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I 


SherifD.ElWakil 


Processes  and  Design 
for  Manufacturing 


Second 


Processes  and  Design 
for  Manufacturing 

Second  Edition 


SherifD.  ElWakil 

University  of  Massachusetts  Dartmouth 


WAVELAND 


PRESS,  INC. 


Prospect  Heights,  Illinois 


To  the  memory  ofMamdouh  El-Wakil,  M.D.,  Ph.D. 


For  information  about  this  book,  contact: 
Waveland  Press,  Inc. 
P.O.  Box  400 

Prospect  Heights,  Illinois  60070 
(847)634-0081 
www.waveland.com 


Copyright  ©  1998  by  Sherif  D.  El  Wakil 
2002  reissued  by  Waveland  Press,  Inc. 

ISBN  1-57766-255-5 

All  rights  reserved.  No  part  of  this  book  may  be  reproduced,  stored  in  a  retrieval  system,  or  transmitted  in 
any  form  or  by  any  means  without  permission  in  writing  from  the  publisher. 

Printed  in  the  United  States  of  America 

7     6     5     4     3     2      1 


Chapter  1    Overview    1 

INTRODUCTION 

1.1  Definition  of  Manufacturing     1 

1.2  Relationship  Between  Manufacturing  and  Standard  of  Living     2 

1.3  Overview  of  the  Manufacturing  Processes     2 

1.4  Types  of  Production     3 

1.5  Fundamentals  of  Manufacturing  Accuracy     4 

1.6  The  Production  Turn     6 

1.7  Product  Life  Cycle     7 

1.8  Technology  Development  Cycle     8 

1.9  The  Design  Process     10 

1.10  Product  Design:  The  Concept  of  Design  for  Manufacturing     14 
Review  Questions     16 

Chapter  2    Concurrent  Engineering     17 

INTRODUCTION 

2.1  Reasons  for  Adopting  Concurrent  Engineering     19 

2.2  Benefits  of  Concurrent  Engineering     20 

2.3  Factors  Preventing  the  Adoption  of  Concurrent  Engineering     2 1 

2.4  The  Four  Pillars  of  Concurrent  Engineering     22 

2.5  Forces  of  Change     24 

2.6  A  Success  Story:  Nippondenso     30 
Review  Questions    32 


iii 


iv  Contents 

Chapter  3    Casting  and  Foundry  Work     33 

I NTRODUCTION 

3.1  Classifications  of  Casting  by  Mold  Material     34 

3.2  Classifications  of  Casting  by  Method  of  Filling  the  Mold     52 

3.3  Classifications  of  Casting  by  Metal  to  be  Cast    58 

3.4  Foundry  Furnaces     63 

3.5  Casting  Defects  and  Design  Considerations     68 

3.6  Cleaning,  Testing,  and  Inspection  of  Castings     72 

3.7  Castability  (Fluidity)     75 
Review  Questions     76 
Design  Example     78 
Design  Projects    81 

Chapter  4    Joining  of  Metals     84 

I NTRODUCTION 

4.1  Riveting     84 

4.2  Welding     84 

4.3  Surfacing  and  Hard-Facing     120 

4.4  Thermal  Cutting  of  Metals     121 

4.5  Brazing  and  Soldering     123 

4.6  Sticking  of  Metals     128 
Review  Questions    130 
Problems     133 
Design  Example     133 
Design  Projects    137 

Chapter  5    Metal  Forming     139 

I  NTRODUCTION 

5.1  Plastic  Deformation     140 

5.2  Rolling     145 

5.3  Metal  Drawing     155 

5.4  Extrusion     158 

5.5  Forging     176 

5.6  Cold  Forming  Processes     201 
Review  Questions    204 
Problems     207 

Design  Example    207 
Design  Projects    209 


Contents 

Chapter  6    Sheet  Metal  Working     211 

INTRODUCTION 

6.1  Press  Working  Operations     212 

6.2  High-Energy-Rate  Forming  (HERF)     238 

6.3  Spinning  of  Sheet  Metal     241 
Review  Questions    242 
Problems     244 

Design  Example    245 
Design  Projects    246 

Chapter  7    Powder  Metallurgy     248 

INTRODUCTION 

7.1  Metal  Powders     249 

7.2  Powder  Metallurgy:  The  Basic  Process     254 

7.3  Operational  Flowchart     258 

7.4  Alternative  Consolidation  Techniques     258 

7.5  Secondary  Consolidation  Operations     263 

7.6  Finishing  Operations     264 

7.7  Porosity  in  Powder  Metallurgy  Parts     266 

7.8  Design  Considerations  for  Powder  Metallurgy  Parts     268 

7.9  Advantages  and  Disadvantages  of  Powder  Metallurgy     270 

7.10  Applications  of  Powder  Metallurgy  Parts     270 
Review  Questions    274 

Problems     275 
Design  Project    277 

Chapter  8    Plastics     278 

I  NTRODUCTION 

8.1  Classification  of  Polymers     279 

8.2  Properties  Characterizing  Plastics  and  Their  Effect  on  Product  Design     282 

8.3  Polymeric  Systems     283 

8.4  Processing  of  Plastics     291 

8.5  Fiber-Reinforced  Polymeric  Composites     303 
References     328 

Review  Questions    328 
Design  Projects     330 


vi  Contents 

Chapter  9    Physics  of  Metal  Cutting     331 

INTRODUCTION 

9.1  Cutting  Angles     332 

9.2  Chip  Formation     334 

9.3  Cutting  Forces     339 

9.4  Oblique  Versus  Orthogonal  Cutting     343 

9.5  Cutting  Tools     348 

9.6  Machinability     353 

9.7  Cutting  Fluids     354 

9.8  Chatter  Phenomenon     356 

9.9  Economics  of  Metal  Cutting     356 
Review  Questions    358 
Problems     359 

Design  Project    360 

Chapter  10    Machining  of  Metals     361 

INTRODUCTION 

10.1  Turning  Operations     362 

10.2  Shaping  and  Planing  Operations     379 

10.3  Drilling  Operations     382 

10.4  Milling  Operations     392 

10.5  Grinding  Operations     400 

10.6  Sawing  Operations     405 

10.7  Broaching  Operations     407 

10.8  Nontraditional  Machining  Operations     408 
Review  Questions    411 

Problems     413 

Chapter  11    Product  Cost  Estimation     415 

INTRODUCTION 

11.1  Costs:  Classification  and  Terminology     416 

11.2  Labor  Cost  Analysis     418 

11.3  Material  Cost  Analysis     421 

11.4  Equipment  Cost  Analysis     423 

11.5  Engineering  Cost     425 

11.6  Overhead  Costs     425 

11.7  Design  to  Cost    427 


Contents  vii 

Review  Questions    427 
Problems     428 
Design  Project    430 

Chapter  12    Design  for  Assembly     431 

INTRODUCTION 

12.1  Types  and  Characteristics  of  Assembly  Methods     432 

12.2  Selection  of  Assembly  Method     435 

12.3  Product  Design  for  Manual  Assembly     436 

12.4  Product  Design  for  Automatic  Assembly     438 

12.5  Product  Design  for  Robotic  Assembly     445 

12.6  Methods  for  Evaluating  and  Improving  Product  DFA     446 
Review  Questions    459 

Design  Project     459 

Chapter  13    Environmentally  Conscious  Design 
and  Manufacturing    460 

INTRODUCTION 

13.1  Solid- Waste  Sources     462 

13.2  Solid-Waste  Management     464 

13.3  Guidelines  for  Environmentally  Conscious  Product  Design     469 

13.4  Environmentally  Conscious  Manufacturing     472 

13.5  Environmental  Protection  and  Pollution  Control  Legislation     473 
Review  Questions    475 

Chapter  14    Computer-Aided  Manufacturing     476 

INTRODUCTION 

14.1  Numerical  Control  (NC)     476 

14.2  Computerized  Numerical  Control  (CNC)     494 

14.3  Direct  Numerical  Control  (DNC)     498 

14.4  Computer-Aided  Part  Programming     499 

14.5  Other  Applications  of  Computer- Aided  Manufacturing     514 
Review  Questions    516 

Problems     518 

Chapter  14  Appendix     520 


viii  Contents 

Chapter  15    Industrial  Robots     523 

INTRODUCTION 

15.1  Reasons  for  Using  Robots     524 

15.2  Methods  for  Classifying  Robots     525 

15.3  Components  of  a  Robot     536 

15.4  End  Effectors     537 

15.5  Sensors     540 

15.6  Industrial  Applications  of  Robots     541 
Review  Questions    545 

Chapter  16    Automated  Manufacturing  Systems     547 

INTRODUCTION 

16.1  Computer-Integrated  Manufacturing  (CIM)     548 

16.2  Group  Technology  (GT)     556 

16.3  Computer-Aided  Process  Planning  (CAPP)     562 

16.4  Material-Requirement  Planning  (MRP)     565 

16.5  The  Potential  of  Artificial  Intelligence  in  Manufacturing     566 

16.6  Flexible  Manufacturing  System  (FMS)     568 
Review  Questions    575 

Appendix    Materials  Engineering     577 

I  NTRODUCTION 

A.l  Types  of  Materials     577 

A.2  Properties  of  Materials     580 

A.3  Standard  Tests  for  Obtaining  Mechanical  Properties     580 

A.4  Phase  Diagrams     590 

A.5  Ferrous  Alloys     595 

A.6  Aluminum  Alloys     603 

A.7  Copper  Alloys     604 

References     605 

Index     610 


At  the  time  the  author's  first  book  on  processes  and  design  for  manufacturing  was 
published,  the  main  concern  of  the  manufacturing/engineering  academic  commu- 
nity was  the  erroneous  picture  of  manufacturing  as  involving  little  more  than  manual 
training  (i.e.,  manual  skills  acquired  by  on-site  training  in  machine  shops  and  the  like). 
Unfortunately,  this  distorted  view  of  manufacturing  was  created  and  fueled  by  the 
shallow,  descriptive,  and  qualitative  manner  in  which  the  vast  majority  of  books  then 
covered  the  subject.  Now,  design  for  manufacturing  is  a  "hot  topic,"  and  engineers  in 
all  disciplines  are  beginning  to  realize  its  strategic  importance.  Many  government  pro- 
grams are  aimed  at  enhancing  the  efficiency  of  product  development  and  design.  The 
present  text  serves  to  provide  engineering  students  with  the  knowledge  and  skills  re- 
quired for  them  to  become  good  product  designers. 

The  design  component  in  this  book  has  been  strengthened  by  adding  four  new 
chapters: 

•  Chapter  2,  Concurrent  Engineering 

•  Chapter  1 1 ,  Product  Cost  Estimation 

•  Chapter  12,  Design  for  Assembly 

•  Chapter  13,  Environmentally  Conscious  Design  and  Manufacturing 

Also,  whenever  applicable,  chapters  have  been  supplemented  by  design  examples  il- 
lustrating the  interaction  between  design  and  manufacturing  and  showing  how  prod- 
ucts can  be  designed  for  producibility,  taking  factors  like  the  lot  size  into 
consideration.  In  addition,  some  design  projects,  which  were  previously  assigned  at 
the  University  of  Massachusetts  Dartmouth,  have  been  given  at  the  end  of  several 
chapters.  Students  are  encouraged  to  use  computational  tools  like  spreadsheets  and 
other  software  for  modeling  and  analysis. 

The  text  has  also  been  supplemented  with  an  appendix  that  covers  the  fundamen- 
tals of  materials  engineering.  It  provides  a  basis  for  understanding  manufacturing 
processes,  as  well  as  for  selecting  materials  during  the  product  design  process.  It  is 


IX 


Preface 


aimed  at  engineering  students  who  have  not  taken  materials  science  as  a  prerequisite 
for  a  course  on  manufacturing  processes  but  is  not  meant  as  a  substitute  for  any  mate- 
rials science  textbook. 

The  author  wishes  to  acknowledge  the  contributions  of  the  many  corporations  and 
individuals  who  supplied  various  figures  and  photographs  or  provided  software  to  aid 
in  producing  this  book,  chief  among  them  Silverscreen.  Thanks  are  also  extended  to 
reviewers  of  the  manuscript: 

Mary  C.  Kocak,  Pellissippi  State  Technical  Community  College 

Zhongming  (Wilson)  Liang,  Purdue  University — Fort  Wayne 

Wen  F.  Lu,  University  of  Missouri — Rolla 

Antonio  Minardi,  University  of  Central  Florida 

Charles  Mosier,  Clarkson  University 

Masud  Salimian,  Morgan  State  University 

Richard  D.  Sisson.  Jr.,  Worcester  Polytechnic  Institute 

Joel  W.  Troxler,  Montana  State  University 

David  C.  Zenger,  Worcester  Polytechnic  Institute 

Yuming  Zhang,  University  of  Kentucky 

A  note  of  gratitude  also  goes  to  Ana  Gonzalez  for  her  hard  work  in  typing  the 
manuscript.  The  author  wishes  to  thank  Andrea  Goldman  and  Jean  Peck  for  their 
encouragement  and  support.  Finally,  the  author  must  express  his  profound  gratitude  to 
his  wife  and  children  for  their  patience  as  the  huge  task  of  completing  this  second  edi- 
tion unfolded.  God  knows  the  sacrifice  they  gave. 

SherifD.  El  Wakil 
North  Dartmouth,  Massachusetts 


Chapter  1 


vervlew 


INTRODUCTION 

Before  learning  about  various  manufacturing  processes  and  the  concept  of  de- 
sign for  manufacturing,  we  first  must  become  familiar  with  some  technical  terms 
that  are  used  frequently  during  the  planning  for  and  operation  of  industrial  man- 
ufacturing plants.  We  also  must  understand  thoroughly  the  meaning  of  each  of 
these  terms,  as  well  as  their  significance  to  manufacturing  engineers.  The  ex- 
planation of  the  word  manufacturing  and  its  impact  on  the  life-style  of  the  peo- 
ple of  industrialized  nations  should  logically  come  at  the  beginning.  In  fact,  this 
chapter  will  cover  all  these  issues  and  also  provide  a  better  understanding  of  the 
design  process,  as  well  as  the  different  stages  involved  in  it.  Finally,  the  concept 
of  design  for  manufacturing  and  why  it  is  needed  will  be  explained. 


•    /^~*\ 


1.1  DEFINITION  OF  MANUFACTURING 


Manufacturing  can  be  defined  as  the  transformation  of  raw  materials  into  useful  prod- 
ucts through  the  use  of  the  easiest  and  least  expensive  methods.  It  is  not  enough,  there- 
fore, to  process  some  raw  materials  and  obtain  the  desired  product.  It  is,  in  fact,  of  major 
importance  to  achieve  this  goal  by  employing  the  easiest,  fastest,  and  most  efficient 
methods.  If  less  efficient  techniques  are  used,  the  production  cost  of  the  manufactured 
part  will  be  high,  and  the  part  will  not  be  as  competitive  as  similar  parts  produced  by 
other  manufacturers.  Also,  the  production  time  should  be  as  short  as  possible  in  order  to 
capture  a  larger  market  share. 

The  function  of  a  manufacturing  engineer  is,  therefore,  to  determine  and  define  the 
equipment,  tools,  and  processes  required  to  convert  the  design  of  the  desired  product 
into  reality  in  an  efficient  manner.  In  other  words,  it  is  the  engineer's  task  to  find  out 
the  most  appropriate,  optimal  combination  of  machinery,  materials,  and  methods 


Overview 

needed  to  achieve  economical  and  trouble-free  production.  Thus,  a  manufacturing  en- 
gineer must  have  a  strong  background  in  materials  and  up-to-date  machinery,  as  well 
as  the  ability  to  develop  analytical  solutions  and  alternatives  for  the  open-ended  prob- 
lems experienced  in  manufacturing.  An  engineer  must  also  have  a  sound  knowledge  of 
the  theoretical  and  practical  aspects  of  the  various  manufacturing  methods. 


..2  RELATIONSHIP  BETWEEN  MANUFACTURING 
AND  STANDARD  OF  LIVING 

The  standard  of  living  in  any  nation  is  reflected  in  the  products  and  services  avail- 
able to  its  people.  In  a  nation  with  a  high  standard  of  living,  a  middle-class  family 
usually  owns  an  automobile,  a  refrigerator,  an  electric  stove,  a  dishwasher,  a  wash- 
ing machine,  a  vacuum  cleaner,  a  stereo,  and,  of  course,  a  television  set.  Such  a 
family  also  enjoys  health  care  that  involves  modern  equipment  and  facilities.  All 
these  goods,  appliances,  and  equipment  are  actually  raw  materials  that  have  been 
converted  into  manufactured  products.  Therefore,  the  more  active  in  manufacturing 
raw  materials  the  people  of  a  nation  are,  the  more  plentiful  those  goods  and  ser- 
vices become;  as  a  consequence,  the  standard  of  living  of  the  people  in  that  nation 
attains  a  high  level.  On  the  other  hand,  nations  that  have  raw  materials  but  do  not 
fully  exploit  their  resources  by  manufacturing  those  materials  are  usually  poor  and 
are  considered  to  be  underdeveloped.  It  is,  therefore,  the  know-how  and  capability 
of  converting  raw  materials  into  useful  products,  not  just  the  availability  of  miner- 
als or  resources  within  its  territorial  land,  that  basically  determines  the  standard  of 
living  of  a  nation.  In  fact,  many  industrial  nations,  such  as  Japan  and  Switzerland, 
import  most  of  the  raw  materials  that  they  manufacture  and  yet  still  maintain  a  high 
standard  of  living. 


VERVIEW  OF  THE 
MANUFACTURING  PROCESSES 

The  final  desired  shape  of  a  manufactured  component  can  be  achieved  through  one  or 
more  of  the  following  four  approaches: 

1.  Changing  the  shape  of  the  raw  stock  without  adding  material  to  it  or  taking  mater- 
ial away  from  it.  Such  change  in  shape  is  achieved  through  plastic  deformation,  and 
the  manufacturing  processes  that  are  based  on  this  approach  are  referred  to  as  metal 
forming  processes.  These  processes  include  bulk  forming  processes  like  rolling, 
extrusion,  forging,  and  drawing,  as  well  as  sheet  metal  forming  operations  like 
bending,  deep  drawing,  and  embossing.  Bulk  forming  operations  are  covered  in 
Chapter  5,  and  the  working  of  sheet  metal  is  covered  in  Chapter  6. 

2.  Obtaining  the  required  shape  by  adding  metal  or  joining  two  metallic  parts  to- 
gether, as  in  welding,  brazing,  or  metal  deposition.  These  operations  are  covered 
in  Chapter  4. 


1.4  Types  of  Production  3 

3.  Molding  molten  or  particulate  metal  into  a  cavity  that  has  the  same  shape  as  the 
final  desired  product,  as  in  casting  and  powder  metallurgy.  These  processes  are  cov- 
ered in  Chapters  3  and  7,  respectively. 

4.  Removing  portions  from  the  stock  material  to  obtain  the  final  desired  shape.  A  cut- 
ting tool  that  is  harder  than  the  stock  material  and  possesses  certain  geometric  char- 
acteristics is  employed  in  removing  the  undesired  material  in  the  form  of  chips. 
Several  chip-making  (machining)  operations  belong  to  this  group.  They  are  exem- 
plified by  turning,  milling,  and  drilling  operations  and  are  covered  in  Chapter  10. 
The  physics  of  the  process  of  chip  removal  is  covered  in  Chapter  9. 


YPES  OF  PRODUCTION 


Modern  industries  can  be  classified  in  different  ways.  These  classifications  may  be  by 
process,  or  by  product,  or  based  on  production  volume  and  the  diversity  of  products. 
Classification  by  process  is  exemplified  by  casting  industries,  stamping  industries,  and 
the  like.  Classification  by  product  indicates  that  industries  may  belong  to  the  automo- 
tive, aerospace,  and  electronics  groups.  Classification  based  on  production  volume 
identifies  three  distinct  types  of  production:  mass,  job  shop,  and  moderate.  Let  us 
briefly  discuss  the  features  and  characteristics  of  each  type.  We  will  also  discuss  the 
subjects  in  greater  depth  later  in  the  text. 

Mass  Production 

Mass  production  is  characterized  by  the  high  production  volume  of  the  same  (or  very 
similar)  parts  for  a  prolonged  period  of  time.  An  annual  production  volume  of  less  than 
50,000  pieces  cannot  usually  be  considered  as  mass  production.  As  you  may  expect,  the 
production  volume  is  based  on  an  established  or  anticipated  sales  volume  and  is  not  di- 
rectly affected  by  the  daily  or  monthly  orders.  The  typical  example  of  mass-produced 
goods  is  automobiles.  Because  that  type  attained  its  modern  status  in  Detroit,  it  is  some- 
times referred  to  as  the  Detroit  type. 

Job  Shop  Production 

Job  shop  production  is  based  on  sales  orders  for  a  variety  of  small  lots.  Each  lot  may 
consist  of  up  to  200  or  more  similar  parts,  depending  upon  the  customer's  needs.  It  is 
obvious  that  this  type  of  production  is  most  suitable  for  subcontractors  who  produce 
varying  components  to  supply  various  industries.  The  machines  employed  must  be 
flexible  to  handle  frequent  variations  in  the  configuration  of  the  ordered  components. 
Also,  the  personnel  employed  must  be  highly  skilled  in  order  to  handle  a  variety  of 
tasks  that  differ  for  the  different  parts  that  are  manufactured. 

Moderate  Production 

Moderate  production  is  an  intermediate  phase  between  the  job  shop  and  the  mass 
production  types.  The  production  volume  ranges  from  10,000  to  20,000  parts,  and 
the  machines  employed  are  flexible  and  multipurpose.  This  type  of  production  is 


1      Overview 

gaining  popularity  in  industry  because  of  an  increasing  market  demand  for  cus- 
tomized products. 


1.5  FUNDAMENTALS  OF 

MANUFACTURING  ACCURACY 


Modern  manufacturing  is  based  on  flow-type  "mass"  assembly  of  components  into 
machines,  units,  or  equipment  without  the  need  for  any  fitting  operations  performed  on 
those  components.  That  was  not  the  case  in  the  early  days  of  the  Industrial  Revolution, 
when  machines  or  goods  were  individually  made  and  assembled  and  there  was  always 
the  need  for  the  "fitter"  with  his  or  her  file  to  make  final  adjustments  before  assembling 
the  components.  The  concepts  of  mass  production  and  interchangeability  came  into 
being  in  1798,  when  the  American  inventor  Eli  Whitney  (born  in  Westboro,  Massa- 
chusetts) contracted  with  the  U.S.  government  to  make  10,000  muskets.  Whitney 
started  by  designing  a  new  gun  and  the  machine  tools  to  make  it.  The  components  of 
each  gun  were  manufactured  separately  by  different  workers.  Each  worker  was  as- 
signed the  task  of  manufacturing  a  large  number  of  the  same  component.  Meanwhile, 
the  dimensions  of  those  components  were  kept  within  certain  limits  so  that  they  could 
replace  each  other  if  necessary  and  fit  their  mating  counterparts.  In  other  words,  each 
part  would  fit  any  of  the  guns  he  made.  The  final  step  was  merely  to  assemble  the  in- 
terchangeable parts.  By  doing  so,  Eli  Whitney  established  two  very  important  concepts 
on  which  modern  mass  production  is  based — namely,  interchangeability  and  fits.  Let 
us  now  discuss  the  different  concepts  associated  with  the  manufacturing  accuracy  re- 
quired for  modern  mass  production  technologies. 

Tolerances 

A  very  important  fact  of  the  manufacturing  science  is  that  it  is  almost  impossible  to  ob- 
tain the  desired  nominal  dimension  when  processing  a  workpiece.  This  is  caused  by 
the  inevitable,  though  very  slight,  inaccuracies  inherent  in  the  machine  tool,  as  well  as 
by  various  complicated  factors  like  the  elastic  deformation  and  recovery  of  the  work- 
piece  and/or  the  fixture,  temperature  effects  during  processing,  and  sometimes  the  skill 
of  the  operator.  Because  it  is  difficult  to  analyze  and  completely  eliminate  the  effects 
of  these  factors,  it  is  more  feasible  to  establish  a  permissible  degree  of  inaccuracy  or  a 
permissible  deviation  from  the  nominal  dimension  that  would  not  affect  the  proper 
functioning  of  the  manufactured  part  in  any  detrimental  way.  According  to  the  Inter- 
national Standardization  Organization  (ISO)  system,  the  nominal  dimension  is  referred 
to  as  the  basic  size  of  the  part. 

The  deviations  from  the  basic  size  to  each  side  (in  fact,  both  can  also  be  on  the 
same  side)  determine  the  high  and  the  low  limits,  respectively,  and  the  difference  be- 
tween these  two  limits  of  size  is  called  the  tolerance.  It  is  an  absolute  value  without  a 
sign  and  can  also  be  obtained  by  adding  the  absolute  values  of  the  deviations.  As  you 
may  expect,  the  magnitude  of  the  tolerance  is  dependent  upon  the  basic  size  and  is  des- 


1.5  Fundamentals  of  Manufacturing  Accuracy 


FIGURE    1.1 

The  relationship 
between  tolerance 
and  production  cost 


Tolerance 


ignated  by  an  alphanumeric  symbol  called  the  grade.  There  are  1 8  standard  grades  of 
tolerance  in  the  ISO  system,  and  the  tolerances  can  be  obtained  from  the  formulas  or 
the  tables  published  by  the  ISO. 

Smaller  tolerances,  of  course,  require  the  use  of  high-precision  machine  tools  in  man- 
ufacturing the  parts  and,  therefore,  increase  production  cost.  Figure  1 . 1  indicates  the  rela- 
tionship between  the  tolerance  and  the  production  cost.  As  can  be  seen,  very  small  toler- 
ances necessitate  very  high  production  cost.  Therefore,  small  tolerances  should  not  be 
specified  when  designing  a  component  unless  they  serve  a  certain  purpose  in  that  design. 

Fits 

Before  two  components  are  assembled  together,  the  relationship  between  the  dimensions 
of  the  mating  surfaces  must  be  specified.  In  other  words,  the  location  of  the  zero  line  (i.e., 
the  line  indicating  the  basic  size)  to  which  deviations  are  referred  must  be  established  for 
each  of  the  two  mating  surfaces.  As  can  be  seen  in  Figure  1 .2a,  this  determines  the  degree 
of  tightness  or  freedom  for  relative  motion  between  the  mating  surfaces.  Figure  1 .2a  also 
shows  that  there  are  basically  three  types  of  fits:  clearance,  transition,  and  interference. 

In  all  cases  of  clearance  fit,  the  upper  limit  of  the  shaft  is  always  smaller  than  the 
lower  limit  of  the  mating  hole.  This  is  not  the  case  in  interference  fit,  where  the  lower  limit 
of  the  shaft  is  always  larger  than  the  upper  limit  of  the  hole.  The  transition  fit,  as  the  name 
suggests,  is  an  intermediate  fit.  According  to  the  ISO,  the  internal  enveloped  part  is  always 


FIGURE    1.2 

The  two  systems  of  fit 
according  to  the  ISO: 

(a)  shaft-basis  system; 

(b)  hole-basis  system 


Basic  size 


(a) 


(b) 


L. 


~ 


Hole  tolerance  zone 


3 


Shaft  tolerance  zone 


Overview 

referred  to  as  the  shaft,  whereas  the  surrounding  surface  is  referred  to  as  the  hole.  Accord- 
ingly, from  the  fits  point  of  view,  a  key  is  the  shaft  and  the  key  way  is  the  hole. 

It  is  clear  from  Figures  1.2a  and  b  that  there  are  two  ways  for  specifying  and 
expressing  the  various  types  of  fits:  the  shaft-basis  and  the  hole-basis  systems.  The 
location  of  the  tolerance  zone  with  respect  to  the  zero  line  is  indicated  by  a  letter, 
which  is  always  capitalized  for  holes  and  lowercased  for  shafts,  whereas  the  tolerance 
grade  is  indicated  by  a  number,  as  previously  explained.  Therefore,  a  fit  designation 
can  be  H7/h6,  F6/g5,  or  any  other  similar  form. 

Interchangeability 

When  the  service  life  of  an  electric  bulb  is  over,  all  you  do  is  buy  a  new  one  and  re- 
place the  bulb.  This  easy  operation,  which  does  not  need  a  fitter  or  a  technician,  would 
not  be  possible  without  the  two  main  concepts  of  interchangeability  and  standardiza- 
tion. Interchangeability  means  that  identical  parts  must  be  able  to  replace  each  other, 
whether  during  assembly  or  subsequent  maintenance  work,  without  the  need  for  any 
fitting  operations.  Interchangeability  is  achieved  by  establishing  a  permissible  toler- 
ance, beyond  which  any  further  deviation  from  the  nominal  dimension  of  the  part  is 
not  allowed.  Standardization,  on  the  other  hand,  involves  limiting  the  diversity  and 
total  number  of  varieties  to  a  definite  range  of  standard  dimensions.  An  example  is  the 
standard  gauge  system  for  wires  and  sheets.  Instead  of  having  a  very  large  number  of 
sheet  thicknesses  in  steps  of  0.001  inch,  the  number  of  thicknesses  produced  is  limited 
to  only  45  (in  U.S.  standards).  As  you  can  see  from  this  example,  standardization  has 
far-reaching  economic  implications  and  also  promotes  interchangeability.  Obviously, 
the  engineering  standards  differ  for  different  countries  and  reflect  the  quality  of  tech- 
nology and  the  industrial  production  in  each  case.  Germany  established  the  DIN 
(Deutsche  Ingenieure  Normen),  standards  that  are  finding  some  popularity  worldwide. 
The  former  Soviet  Union  adopted  the  GOST,  standards  that  were  suitable  for  the  pe- 
riod of  industrialization  of  that  country. 


THE  PRODUCTION  TURN 


In  almost  all  cases,  the  main  goal  of  a  manufacturing  project  is  to  make  a  profit,  the  ex- 
ception being  projects  that  have  to  do  with  the  national  security  or  prestige.  Let  us  es- 
tablish a  simplified  model  that  illustrates  the  cash  flow  through  the  different  activities 
associated  with  manufacturing  so  that  we  can  see  how  to  maximize  the  profit.  As  shown 
in  Figure  1.3,  the  project  starts  by  borrowing  money  from  a  bank  to  purchase  machines 
and  raw  materials  and  to  pay  the  salaries  of  the  engineers  and  other  employees.  Next,  the 
raw  materials  are  converted  into  products,  which  are  the  output  of  the  manufacturing  do- 
main. Obviously,  those  products  must  be  sold  (through  the  marketing  department)  in 
order  to  get  cash.  This  cash  is,  in  turn,  used  to  cover  running  costs,  as  well  as  required 
payment  to  the  bank;  any  surplus  money  left  is  the  profit. 

We  can  see  in  this  model  that  the  sequence  of  events  forms  a  continuous  cycle  (i.e., 
a  closed  circuit).  This  cycle  is  usually  referred  to  as  the  production  turn.  We  can  also  re- 
alize the  importance  of  marketing,  which  ensures  the  continuity  of  the  cycle.  If  the  prod- 


1.7  Product  Life  Cycle 


FIGURE    1.3 

Initial  money 

The  production  turn 

Bank 

borrowed  from  bank 

Manufacturing 
plant 

Products 

Products 

Money  to  purchase 

raw  materials  and 

for  the  running  cost 

Money 

payment 
+  profit 

Marketing 
department 

ucts  are  not  sold,  the  cycle  is  obviously  interrupted.  Moreover,  we  can  see  that  maxi- 
mum profit  is  obtained  through  maximizing  the  profit  per  turn  and/or  increasing  the 
number  of  turns  per  year  (i.e.,  running  the  cycle  faster).  Evidently,  these  two  conditions 
are  fulfilled  when  products  are  manufactured  in  the  easiest  and  least  expensive  way. 


1.7  PRODUCT  LIFE  CYCLE 


It  has  been  observed  that  all  products,  from  the  sales  viewpoint,  go  through  the  same 
product  life  cycle,  no  matter  how  diverse  or  dissimilar  they  are.  Whether  the  product 
is  a  new-model  airplane  or  a  coffeemaker,  its  sales  follow  a  certain  pattern  or  sequence 
from  the  time  it  is  introduced  in  the  market  to  the  time  it  is  no  longer  sold.  The  main 
difference  between  the  cycles  of  these  two  products  is  the  span  or  duration  of  the 
cycle,  which  always  depends  upon  the  nature  and  uses  of  the  particular  product.  As  we 
will  see  later  when  discussing  concurrent  engineering  in  Chapter  2,  it  is  very  important 
for  the  designer  and  the  manufacturing  engineer  to  fully  understand  that  cycle  in  order 
to  maximize  the  profits  of  the  production  plant. 

It  is  clear  from  Figure  1 .4  that  the  sales,  as  well  as  the  rate  of  increase  in  sales,  are  ini- 
tially low  during  the  introduction  stage  of  the  product  life  cycle.  The  reason  is  that  the  con- 
sumer is  not  aware  of  the  performance  and  the  unique  characteristics  of  the  product. 


FIGURE    1.4 

The  product  life  cycle 


Decline 


Time 


Overview 

Through  television  and  newspaper  advertisements  and  word-of-mouth  communication,  a 
growing  number  of  consumers  learn  about  the  product  and  its  capabilities.  Meanwhile, 
the  management  works  on  improving  the  performance  and  eliminating  the  shortcomings 
through  minor  design  modifications.  It  is  also  the  time  for  some  custom  tailoring  of  the 
product  for  slightly  different  customer  needs,  in  order  to  serve  a  wider  variety  of  con- 
sumers. As  a  result,  the  customer  acceptance  is  enhanced,  and  the  sales  accordingly  in- 
crease at  a  remarkable  rate  during  this  stage,  which  is  known  as  the  growth  stage. 
However,  this  trend  does  not  continue  forever,  and,  at  a  certain  point,  the  sales  level  out. 
This  is,  in  fact,  the  maturity  stage  of  the  life  cycle.  During  this  stage,  the  product  is  usu- 
ally faced  with  fierce  competition,  but  the  sales  will  continue  to  be  stable  if  the  manage- 
ment succeeds  in  reducing  the  cost  of  the  product  and/or  developing  new  applications  for 
it.  The  more  successful  the  management  is  in  achieving  this  goal,  the  longer  the  duration 
of  the  maturity  stage  will  be.  Finally,  the  decline  stage  begins,  the  sales  fall  at  a  noticeable 
rate,  and  the  product  is,  at  some  point,  completely  abandoned.  The  decrease  in  the  sales  is 
usually  due  to  newer  and  better  products  that  are  pumped  into  the  market  by  competing 
manufacturers  to  serve  some  customer  need.  It  can  also  be  caused  by  diminishing  need  for 
the  uses  and  applications  of  such  a  product.  A  clever  management  would  start  developing 
and  marketing  a  new  product  (B)  during  the  maturity  stage  of  the  previous  one  (A)  so  as 
to  keep  sales  continuously  high,  as  shown  in  Figure  1.5. 


FIGURE    1.5 

The  proper  overlap  of 
products'  life  cycles 


® 


\ 


A 


Time 


1.8  TECHNOLOGY  DEVELOPMENT  CYCLE 


Every  now  and  then,  a  new  technology  emerges  as  result  of  active  research  and  devel- 
opment (R  &  D)  and  is  then  employed  in  the  design  and  manufacture  of  several  differ- 
ent products.  It  can,  therefore,  be  stated  that  technology  is  concerned  with  the  industrial 
and  everyday  applications  of  the  results  of  the  theoretical  and  experimental  studies  that 
are  referred  to  as  engineering.  Examples  of  modern  technologies  include  transistor,  mi- 
crochip, and  fiber  optics. 

The  relationship  between  the  effectiveness  or  performance  of  a  certain  technology 
and  the  effort  spent  to  date  to  achieve  such  performance  is  shown  graphically  in  Fig- 


1.8  Technology  Development  Cycle  9 

ure  1.6.  This  graphical  representation  is  known  as  the  technology  development  cycle. 
It  is  also  sometimes  referred  to  as  the  S  curve  because  of  its  shape.  As  can  be  seen  in 
Figure  1.6,  a  lot  of  effort  is  required  to  produce  a  sensible  level  of  performance  at  the 
early  stage.  Evidently,  there  is  a  lack  of  experimental  experience  since  the  techniques 
used  are  new.  Next,  the  rate  of  improvement  in  performance  becomes  exponential,  a 
trend  that  is  observed  with  almost  all  kinds  of  human  knowledge.  At  some  point,  how- 
ever, the  rate  of  progress  becomes  linear  because  most  ideas  are  in  place;  any  further 
improvement  comes  as  a  result  of  refining  the  existing  ideas  rather  than  adding  new 
ones.  Again,  as  time  passes,  the  technology  begins  to  be  "exhausted,"  and  performance 
levels  out.  A  "ceiling"  is  reached,  above  which  the  performance  of  the  existing  current 
technology  cannot  go  because  of  social  and/or  technological  considerations. 

An  enlightened  management  of  a  manufacturing  facility  would  allocate  resources 
and  devote  effort  to  an  active  R&D  program  to  come  up  with  a  new  technology  (B)  as 
soon  as  it  realized  that  the  technology  on  which  the  products  are  based  (A)  was  beginning 
to  mature.  The  production  activities  would  then  be  transferred  to  another  S  curve,  with  a 
higher  ceiling  for  performance  and  greater  possibilities,  as  shown  in  Figure  1 .7.  Any  delay 
in  investing  in  R  &  D  for  developing  new  technology  may  result  in  creating  a  gap  between 
the  two  curves  (instead  of  continuity  with  the  overlap  shown  in  Figure  1.7),  with  the  final 
outcome  being  to  lose  the  market  to  competing  companies  that  possess  newer  technology. 
In  fact,  the  United  States  dominated  the  market  of  commercial  airliners  because  compa- 
nies like  Boeing  and  McDonnell  Douglas  knew  exactly  when  to  switch  from  propeller- 
driven  airplanes  to  jet-propulsion  commercial  airliners.  This  is  contrary  to  what  some 
major  computer  companies  did  when  they  continued  to  develop  and  produce  mainframe 
computers  and  did  not  recognize  when  to  make  the  switch  to  personal  computers.  Current 
examples  of  technological  discontinuity  include  the  change  from  conventional  telecom- 
munications cables  to  fiber  optics  for  communication  and  information  transfer. 


FIGURE    1.6 

The  technology 
development  cycle 
(or  S  curve) 


Effort 


10 


1      Overview 


FIGURE    1.7 

Transfer  from  one  S 
curve  to  another 


Effort 


THE  DESIGN  PROCESS 


An  engineer  is  a  problem  solver  who  employs  his  or  her  scientific  and  empirical 
knowledge  together  with  inventiveness  and  expert  judgment  to  obtain  solutions  for 
problems  arising  from  societal  needs.  These  needs  are  usually  satisfied  by  some  phys- 
ical device,  structure,  or  process.  The  creative  process  by  which  one  or  more  of  the 
fruits  of  the  engineer's  effort  are  obtained  is  referred  to  as  design.  It  is,  indeed,  the  core 
of  engineering  that  provides  the  professional  engineer  with  the  chance  of  creating  orig- 
inal designs  and  watching  them  become  realities.  The  satisfaction  that  the  engineer 
feels  following  the  implementation  of  his  or  her  design  is  the  most  rewarding  experi- 
ence in  the  engineering  profession.  Because  design  is  created  to  satisfy  a  societal  need, 
there  can  be  more  than  one  way  to  achieve  that  goal.  In  other  words,  several  designs 
can  address  the  same  problem.  Which  one  is  the  best  and  most  efficient  design?  Only 
time  will  tell  because  it  is  actually  the  one  that  would  be  favored  by  the  customers 
and/or  the  society  as  a  whole. 

Although  there  is  no  single  standard  sequence  of  steps  to  create  a  workable  de- 
sign, E.  V.  Krick  has  outlined  the  procedure  involved  in  the  design  process,  and  his 
work  has  gained  widespread  acceptance.  Following  is  a  discussion  of  the  stages  of  the 
design  process  according  to  Krick.  (See  the  references  at  the  back  of  the  book  for  more 
detailed  information.) 

Problem  Formulation 

As  illustrated  in  Figure  1.8,  problem  formulation  is  the  first  stage  of  the  design 
process.  This  phase  comes  as  a  result  of  recognizing  a  problem  and  involves  defining 
that  problem  in  a  broad  perspective  without  getting  deep  into  the  details.  It  is  also  at 
this  stage  that  the  engineer  decides  whether  or  not  the  problem  at  hand  is  worth  solv- 
ing. In  other  words,  this  stage  basically  constitutes  a  feasibility  study  of  the  problem 
arising  from  a  recognized  need.  The  designer  should,  therefore,  realize  the  importance 


1.9  The  Design  Process 


11 


FIGURE    1.8 

The  design  process 
(Adapted  from  Krick,  An 
Introduction  to 
Engineering  and 
Engineering  Design, 
2nd  ed.  New  York:  John 
Wiley,  1969) 


Recognition  of 
a  problem  to 
be  solved 

| 

Problem  formulation 
Problem  analysis 
Search 
Decision 
Documentation 

' 

' 

Completely 

specified 

solution 


The  process 
of  design 


of  this  stage.  Neglecting  it  may  result  in  wasting  money  in  an  effort  to  solve  a  prob- 
lem that  is  not  worth  solving  or  wasting  time  on  details  that  make  it  extremely  difficult 
to  get  a  broad  view  of  the  problem  so  as  to  select  the  appropriate  path  for  solving  it. 
The  formulation  of  a  problem  can  take  any  form  that  is  convenient  to  the  designer,  al- 
though diagrammatic  sketching  (in  particular,  the  black-box  method)  has  proven  to  be 
a  valuable  tool. 


Problem  Analysis 

The  second  stage  involves  much  information  gathering  and  processing  in  order  to 
come  up  with  a  detailed  definition  of  the  problem.  Such  information  may  come  from 
handbooks,  from  manufacturers'  catalogs,  leaflets,  and  brochures,  as  well  as  from  per- 
sonal contacts.  You  are  strongly  advised  to  seek  information  wherever  you  can  find  it; 
workers  at  all  levels  of  a  company  may  have  some  key  information  that  you  can  use. 
The  end  product  should  be  a  detailed  analysis  of  the  qualitative  and  quantitative  char- 
acteristics of  the  input  and  output  variables  and  constraints,  as  well  as  the  criteria  that 
will  be  used  in  selecting  the  best  design. 

Search  for  Alternative  Solutions 

In  the  third  stage,  the  designer  actively  seeks  alternative  solutions.  A  good  practice  is  to 
make  a  neat  sketch  for  each  preliminary  design  with  some  notes  about  its  pros  and  cons. 
All  sketches  should  be  kept  even  after  a  different  final  design  is  selected  so  that  if  that 


12  1      Overview 


final  design  is  abandoned  for  some  reason,  a  designer  does  not  have  to  start  from  the  be- 
ginning again.  It  is  also  important  to  remember  not  to  end  the  search  for  alternative  so- 
lutions prematurely,  before  it  is  necessary  or  desirable  to  do  so.  Sometimes,  a  designer 
gets  so  involved  with  details  of  what  he  or  she  thinks  is  a  good  idea  or  solution  that  he 
or  she  will  become  preoccupied  with  these  details,  spending  time  on  them  instead  of 
searching  for  other  good  solutions.  Therefore,  you  are  strongly  advised  to  postpone 
working  out  the  details  until  you  have  an  appropriate  number  of  viable  solutions. 

It  is,  indeed,  highly  recommended  to  employ  collaborative  methods  for  enabling 
the  mind  to  penetrate  into  domains  that  might  otherwise  remain  unexplored.  A  typical 
example  is  the  technique  of  brainstorming,  where  a  few  or  several  people  assemble  to 
produce  a  solution  for  a  problem  by  creating  an  atmosphere  that  encourages  everyone 
to  contribute  with  whatever  comes  to  mind.  After  the  problem  is  explained,  each  mem- 
ber comes  up  with  an  idea  that  is,  in  turn,  recorded  on  a  blackboard,  thus  making  all 
ideas  evident  to  all  team  members. 

Decision  Making 

The  fourth  stage  involves  the  thorough  weighing  and  judging  of  the  different  solutions 
with  the  aim  of  being  able  to  choose  the  most  appropriate  one.  That  is,  trade-offs  have 
to  be  made  during  this  stage.  They  can  be  achieved  by  establishing  a  decision  matrix, 
as  shown  in  Figure  1 .9. 

As  can  be  seen  in  Figure  1 .9,  each  of  the  major  design  objectives  is  in  a  column,  and 
each  solution  is  allocated  a  row.  Each  solution  is  evaluated  with  regard  to  how  it  fulfills 
each  of  the  design  objectives  and  is,  therefore,  given  a  grade  (on  a  scale  of  1  to  10)  in  each 
column.  Because  the  design  objectives  do  not  have  the  same  weight,  each  grade  must  be 
multiplied  by  a  factor  representing  the  weight  of  the  design  function  for  which  it  was 
given.  The  total  of  all  the  products  of  multiplication  is  the  score  of  that  particular  solution 
and  can  be  considered  as  a  true  indication  of  how  that  solution  fulfills  the  design  objec- 
tives. As  you  can  see,  this  technique  provides  a  mechanism  for  rating  the  various  solu- 
tions, thus  eliminating  most  and  giving  further  consideration  to  only  a  few. 

The  chosen  design  is  next  subjected  to  a  thorough  analysis  in  order  to  optimize 
and  refine  it.  Detailed  calculations,  whether  manual  or  computational,  are  involved  at 
this  point.  Both  analytical  and  experimental  modeling  are  also  extensively  employed 
as  tools  in  refining  the  design.  It  is  important,  therefore,  to  now  discuss  modeling  and 
simulation.  A  model  can  be  defined  as  a  simplified  representation  of  a  real-life  situa- 
tion that  aids  in  the  analysis  of  an  associated  problem.  There  are  many  ways  for  clas- 
sifying and  identifying  models.  For  example,  models  can  be  descriptive,  illustrating  a 
real-world  counterpart,  or  prescriptive,  helping  to  predict  the  performance  of  the  actual 
system.  They  can  also  be  deterministic  or  probabilistic  (used  when  making  decisions 
under  uncertainty).  A  simple  example  of  a  model  is  the  free-body  diagram  used  to  de- 
termine the  internal  tensile  force  acting  in  a  wire  with  a  weight  attached  to  its  end. 
There  are  many  computer  tools  (software)  that  are  employed  by  designers  to  create 
models  easily  and  quickly.  Examples  include  geometric  modeling  and  finite  element 
analysis  software  packages.  On  the  other  hand,  simulation  can  be  defined  as  the 
process  of  experimenting  with  a  model  by  subjecting  it  to  various  values  of  input  pa- 


1.9  The  Design  Process 


13 


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14 


Overview 


rameters  and  observing  the  output,  which  can  be  taken  as  an  indication  of  the  behav- 
ior of  the  real-world  system  under  the  tested  conditions.  As  you  can  see,  simulation 
can  save  a  lot  of  time  and  effort  that  could  be  spent  on  experimental  models  and  pro- 
totypes. This  saving  is  particularly  evident  when  computer  simulation  is  employed. 
Still,  simulation  would  not  eliminate  design  iterations  but  rather  would  minimize  their 
number.  You  are,  therefore,  urged  to  make  use  of  these  tools  whenever  possible. 

Documentation 

In  the  fifth  and  last  stage  of  the  design  process,  the  designer  organizes  the  material  ob- 
tained in  the  previous  stage  and  puts  it  in  shape  for  presentation  to  his  or  her  superiors.  The 
output  of  this  stage  should  include  the  attributes  and  performance  characteristics  of  the  re- 
fined design,  given  in  sufficient  detail.  Accordingly,  the  designer  must  communicate  all 
information  in  the  form  of  clear  and  easy-to-understand  documents.  Documentation  con- 
sists of  carefully  prepared,  detailed,  and  dimensioned  engineering  drawings  (i.e.,  assem- 
bly drawings  and  workshop  drawings  or  blueprints),  a  written  report,  and  possibly  an 
iconic  model.  With  the  recent  development  in  rapid  prototyping  techniques,  a  prototype 
can  certainly  be  a  good  substitute  for  an  iconic  model.  This  approach  has  the  advantage  of 
revealing  problems  that  may  be  encountered  during  manufacturing. 


1.10  PRODUCT  DESIGN:  THE  CONCEPT 
OF  DESIGN  FOR  MANUFACTURING 


The  conventional  procedure  for  product  design,  as  illustrated  in  Figure  1.10,  used  to 
start  with  an  analysis  of  the  desired  function,  which  usually  dictated  the  form  as  well 
as  the  materials  of  the  product  to  be  made.  The  design  (blueprint)  was  then  sent  to  the 
manufacturing  department,  where  the  kind  and  sequence  of  production  operations 
were  determined  mainly  by  the  form  and  materials  of  the  product.  In  fact,  the  old  de- 
sign procedure  had  several  disadvantages  and  shortcomings: 


FIGURE    1.10 

The  old  procedure  for 
product  design 


Function 


Form 


Material 


Feedback 


Manufacturing 


1.10  Product  Design:  The  Concept  of  Design  for  Manufacturing 


15 


FIGURE    1.11 

The  new  concept  of  a 
manufacturing  system 
for  achieving  rational 
product  designs 


1.  In  some  cases,  nice-looking  designs  were  impossible  to  make;  in  many  other  cases, 
the  designs  had  to  be  modified  so  that  they  could  be  manufactured. 

2.  Preparing  the  design  without  considering  the  manufacturing  process  to  be  carried  out 
and/or  the  machine  tools  available  would  sometimes  result  in  a  need  for  special- 
purpose,  expensive  machine  tools.  The  final  outcome  was  an  increase  in  the  produc- 
tion cost. 

3.  When  the  required  production  volume  was  large,  parts  had  to  be  specially  designed 
to  facilitate  operations  involved  in  mass  production  (such  as  assembly). 

4.  A  group  of  different  products  produced  by  the  same  manufacturing  process  has 
common  geometric  characteristics  and  features  that  are  dictated  by  the  manufactur- 
ing process  employed  (forgings,  for  example,  have  certain  characteristic  design  fea- 
tures that  are  different  from  those  of  castings,  extrusions,  or  stampings).  Ignoring 
the  method  of  manufacturing  during  the  design  phase  would  undermine  these  char- 
acteristic design  features  and  thus  result  in  impractical  or  faulty  design. 

Because  of  these  reasons  and  also  because  of  the  trend  of  integrating  the  activi- 
ties in  a  manufacturing  corporation,  the  modern  design  procedure  takes  into  consid- 
eration the  method  of  manufacturing  during  the  design  phase.  As  can  be  seen  in 
Figure  1.11,  design,  material,  and  manufacturing  are  three  interactive,  interrelated  el- 
ements that  form  the  manufacturing  system,  whose  prime  inputs  are  conceptual  prod- 
ucts (and/or  functions)  and  whose  outputs  are  manufactured  products.  In  fact,  the 
barriers  and  borders  between  the  design  and  manufacturing  departments  are  fading 


Function 


Manufacturing 
system 


Products 


16 


1     Overview 


out  and  will  eventually  disappear.  The  tasks  of  the  designer  and  those  of  the  manu- 
facturing engineer  are  going  to  be  combined  and  done  by  the  same  person.  It  is, 
therefore,  the  mission  of  this  text  to  emphasize  concepts  like  design  for  manufactur- 
ing and  to  promote  the  systems  approach  for  product  design. 


Review  Questions 


1 


~f 


1.  What  is  the  definition  of  manufacturing? 

2.  Is  there  any  relationship  between  the  status  of 
manufacturing  in  a  nation  and  the  standard  of 
living  of  the  people  in  that  nation?  Explain 
why. 

3.  Explain  the  different  approaches  for  obtaining  a 
desired  shape  and  give  examples  of  some  man- 
ufacturing processes  that  belong  to  each  group. 

4.  List  the  different  types  of  production  and  ex- 
plain the  main  characteristics  of  each.  Also 
mention  some  suitable  applications  for  each 
type. 

5.  Explain  the  meaning  of  the  term  tolerance. 

6.  How  do  we  scientifically  describe  the  tightness 
or  looseness  of  two  mating  parts? 

7.  What  concepts  did  Eli  Whitney  establish  to  en- 
sure trouble-free  running  of  the  mass  produc- 
tion of  multicomponent  products? 

8.  What  is  meant  by  the  production  turn?  What 
role  does  marketing  play  in  this  cycle? 


9.  Using  the  concept  of  production  turn,  how  can 
we  maximize  the  profits  of  a  company  by  two 
different  methods? 


10 


Explain  the  stages  involved  in  the  life  cycle  of 
a  product. 

What  is  the  significance  of  the  product  life 
cycle  during  the  phase  of  planning  for  the  pro- 
duction of  new  products? 

What  is  the  S  curve?  Explain  an  American  suc- 
cess story  in  employing  it. 

Give  some  examples  of  transfer  from  one  tech- 
nology development  curve  to  another. 

What  are  the  stages  involved  in  the  design 
process?  Explain  each  briefly. 

What  is  meant  by  trade-offs?  How  can  these  be 
achieved  during  the  decision-making  stage? 

Explain  the  old  approach  for  product  design. 
What  are  its  disadvantages? 

17.  Explain  the  concept  of  design  for  manufactur- 
ing. Why  is  it  needed  in  modern  industries? 


11 


12 


13 


14. 


15 


16 


Chapter  2 


ncurrent 
gineering 


INTRODUCTION 

Concurrent  engineering  is  a  manufacturing  philosophy  that  involves  managing 
the  product  development  process  with  the  aim  of  getting  new  products  with  the 
highest  quality  at  the  best  competitive  price  in  the  least  time  to  the  market.  It 
has  proven  to  be  a  key  factor  for  the  survival  and,  more  importantly,  for  the 
prosperity  of  companies  that  are  clever  enough  to  adopt  its  methodology  and 
tools  (Motorola  and  Hewlett-Packard  are  good  examples).  In  fact,  many  compa- 
nies can,  in  good  faith,  argue  that  they  have  been  using  this  methodology  in 
one  form  or  another  for  some  time — consider  the  efforts  of  corporations  like 
Xerox,  Hewlett-Packard,  and  Ford  in  the  late  1970s  to  review  and  revise  their 
product  design  practices  versus  those  of  their  foreign  competitors.  An  impor- 
tant milestone  in  the  history  of  concurrent  engineering  is  considered  to  be  the 
report  issued  in  December  1988  by  the  Defense  Advanced  Research  Projects 
Agency  (DARPA)  as  a  result  of  a  study  to  improve  concurrency  in  the  product  de- 
sign process,  a  study  that  lasted  more  than  five  years.  Many  professionals  in 
this  field  rightfully  believe  that  the  DARPA  report  is  the  true  foundation  for  the 
concept  of  concurrent  engineering.  Many  terms  were  and  still  are  (though  to  a 
lesser  degree)  used  to  describe  this  methodology.  Examples  include  team  de- 
sign, simultaneous  engineering,  and  integrated  product  development.  The  term 
concurrent  engineering  was  first  coined  by  the  Institute  for  Defense  Analysis 
(IDA),  which  also  provided  the  following  definition: 

Concurrent  engineering  is  a  systematic  approach  to  integrated,  concur- 
rent design  of  products  and  their  related  processes,  including  manufac- 
ture and  support.  This  approach  is  intended  to  cause  the  developers, 


17 


18  2      Concurrent  Engineering 


from  the  outset,  to  consider  all  elements  of  the  product  life  cycle  from 
concept  through  disposal,  including  quality,  cost,  schedule,  and  user  re- 
quirements. 

A  good  way  to  understand  this  new  concept  and  what  it  means  is  to  compare 
the  product  development  process  in  the  traditional  engineering  approach,  which 
is  usually  referred  to  as  serial  or  sequential  engineering,  with  the  one  in  the  con- 
current engineering  environment.  In  serial  engineering,  a  team  of  qualified  pro- 
fessional engineers  designs  a  product  without  much  interaction  with  or  input  from 
other  departments  within  the  corporation  such  as  manufacturing,  sales,  or  cus- 
tomer service.  A  model  or  prototype  is  then  fabricated  in  the  prototype  workshop 
based  on  the  documented  design  produced  by  the  team.  Note  that  the  environ- 
ment in  the  model  shop  is  ideal  and  is  different  from  the  real  one  on  the  shop  floor 
during  production.  Weeks  or  even  months  after  releasing  the  design,  the  testing 
department  receives  the  model  and  carries  out  acceptance  tests  to  make  sure 
that  the  model  conforms  to  the  documented  design  and  also  meets  the  criteria 
established  and  agreed  upon  for  the  functioning  and  performance  of  the  prod- 
uct. As  you  may  expect,  alterations  and  modification  and/or  revisions  of  the  de- 
sign are  needed  in  most  cases  as  a  result  of  the  absence  of  inputs  from  other 
departments  during  the  design  process.  As  a  consequence,  revisions  call  for 
new  designs  that,  in  turn,  require  the  fabrication  and  testing  of  new  prototypes. 

This  obviously  time-consuming  cycle  may  have  to  be  repeated  a  few  times 
in  order  to  achieve  the  desired  goals.  Such  a  cycle  prolongs  the  new  product 
development  process  to  various  degrees.  Depending  upon  the  complexity  of  the 
product  and  the  number  of  iterations,  the  delay  can  be  excessive,  thus  causing 
damage  to  the  marketing  strategy  and  sales  of  the  new  product.  It  is,  therefore, 
clear  that  the  absence  of  communication  between  the  different  departments 
starting  early  in  the  initial  phase  of  the  design  process  and  continuing  through- 
out that  process  would  result  in  a  larger  number  of  design  iterations  and  de- 
lays in  releasing  the  product  to  the  market.  On  the  contrary,  in  a  concurrent 
engineering  environment,  all  relevant  departments,  such  as  design,  manufac- 
turing, R&D,  and  marketing,  become  involved  and  participate  in  the  design 
process  from  its  very  beginning.  This  interaction  reduces  to  a  large  extent  or 
even  eliminates  design  iterations,  thus  compressing  the  development  cycle 
with  the  final  outcome  of  reducing  the  time-to-market  for  a  new  product.  The 


2.1  Reasons  for  Adopting  Concurrent  Engineering  19 

products  can,  therefore,  be  turned  over  at  a  much  faster  pace.  Bearing  in  mind 
that  the  larger  portion  of  profit  occurs  in  the  early  part  of  the  cycle  for  suc- 
cessful products,  it  would  consequently  be  possible  to  allow  new  products  to 
be  retired  at  nearly  their  optimum  profitability.  Also,  because  customers  are 
consulted  (through  the  marketing  and  customer  service  departments)  early  in 
the  product  development  process,  the  new  product  would  most  probably  pene- 
trate the  market  easily  because  it  would  correctly  meet  customers'  expecta- 
tions in  terms  of  both  function  and  quality.  A  good  example  is  the  case  of  the 
Boeing  777  jetliner.  During  the  initial  development  stage,  Boeing  called  repre- 
sentatives of  its  customers,  including  BOAC  (the  British-government-owned  air- 
lines), although  Britain  is  a  part  of  the  consortium  building  the  air  bus.  By  doing 
so,  Boeing  got  the  praise  and  the  support  of  its  customers  all  over  the  world 
(including  BOAC)  in  the  form  of  orders  of  the  new  product  under  development. 
More  importantly,  some  serious  modifications  in  the  design  were  made  in  the 
very  early  phase,  thus  saving  a  lot  of  money  and  effort  if  they  had  not  been 
done.  The  choice  of  the  height  of  the  wings  above  ground  level  is  a  clear  ex- 
ample. Looking  at  the  initial  conceptual  design,  the  customers  realized  that  the 
wing  height  was  too  high  to  the  extent  that  it  would  create  difficulties  during  fu- 
eling and  would  require  the  use  of  a  special  fueling  truck.  Boeing  was  promptly 
advised  to  lower  the  level  of  the  wings  so  that  the  currently  used  fueling  trucks 
could  be  easily  and  successfully  employed. 


2.1  REASONS  FOR  ADOPTING 
CONCURRENT  ENGINEERING 


Modern  manufacturing  industries  are  facing  many  challenges,  such  as  global  competi- 
tion and  fast-changing  consumer  demands.  These  and  other  challenges  call  for  the 
adoption  of  the  concurrent  engineering  methodology.  Following  is  a  list  of  some  of  the 
challenges  that  can  be  successfully  met  by  concurrent  engineering: 

1.  Increasing  product  complexities  that  prolong  the  product  development  process  and 
make  it  more  difficult  to  predict  the  impact  of  design  decisions  on  the  functionality 
and  performance  of  the  final  product. 

2.  Increasing  global  competitive  pressures  that  result  from  the  emerging  concept  of 
reengineering  (which  enabled  many  Asian  countries  to  produce  extremely  cost- 
effective  products  because  the  cost  of  R  &  D  in  this  case  is  almost  zero).  This  def- 
initely creates  the  need  for  a  cost-effective  product  development  cycle. 


20  2      Concurrent  Engineering 


The  need  for  rapid  response  to  fast-changing  consumer  demands.  This  phenomenon 
calls  for  the  need  to  continuously  listen  to  the  "voice"  of  the  consumer — one  of  the 
solid  principles  of  concurrent  engineering  methodology. 

The  need  for  shorter  product  life  cycles.  This  phenomenon  necessitates  the  intro- 
duction of  new  products  to  the  market  at  a  very  high  pace — something  that  can 
only  be  achieved  by  compressing  the  product  development  cycle.  Consider  the 
changes  that  the  old-fashioned  mechanical  typewriter  has  undergone.  Its  life  cycle 
was  20  to  30  years,  which  then  decreased  to  10  years  for  an  electromechanical 
typewriter  and  finally  to  only  18  months  for  a  word  processor.  This  example  clearly 
illustrates  the  need  to  solve  the  problem  of  time-to-market  pressure. 

Large  organizations  with  several  departments  working  on  developing  numerous 
products  at  the  same  time.  The  amount  of  data  exchanged  between  these  depart- 
ments is  extremely  large,  and  unless  properly  managed  in  a  rational  manner,  the 
flow  and  transfer  of  information  is  not  fast  or  easy  (i.e.,  a  piece  of  information 
needed  by  a  certain  department  may  be  passed  to  another  one).  The  final  outcome 
would  certainly  be  delays  in  the  process  of  product  development  and  products  that 
will  not  appear  in  the  market  at  the  scheduled  time. 

New  and  innovative  technologies  emerging  at  a  very  high  rate,  thus  causing  the 
new  products  to  be  technologically  obsolete  within  a  short  period  of  time.  This  phe- 
nomenon is  particularly  evident  in  the  electronics  industry,  where  the  life  cycle  of 
a  typical  product  is  in  months  (it  used  to  be  years  during  the  1980s).  As  a  conse- 
quence, new  products  must  appear  in  the  market  at  a  very  high  pace  —  something 
that  definitely  necessitates  a  shorter  product  development  cycle. 


2.2  BENEFITS  OF 

CONCURRENT  ENGINEERING 


The  benefits  of  adopting  concurrent  engineering  are  numerous  and  positively  affect  the 
various  activities  in  a  corporation.  Following  is  a  summary  of  the  important  ones: 

1.  Because  the  customer  is  consulted  during  the  early  product  development  process, 
the  product  will  appear  on  the  market  with  a  high  level  of  quality  and  will  meet  the 
expectations  of  the  customer.  The  product  introduction  region  (or  start-up)  of  the 
product  life  cycle  (see  Figure  1.4)  will  be  very  short.  The  sales  volume  will,  there- 
fore, attain  maturity  in  a  very  short  time.  As  a  consequence,  large  revenues  and 
profits  would  be  achieved  during  the  early  phase  of  the  product  life  cycle.  This  is 
very  important  because  products  become  technologically  obsolete  very  quickly  as  a 
result  of  fast-emerging,  innovative  technology. 

2.  Adopting  concurrent  engineering  will  result  in  improved  design  quality,  which  is 
measured  by  the  number  of  design  changes  made  during  the  first  six  months  after 
releasing  a  new  product  to  the  market.  These  design  changes  are  extremely  expen- 
sive unless  caught  early  during  the  product  development  process.  The  lower  the 


2.3  Factors  Preventing  the  Adoption  of  Concurrent  Engineering  21 

number  of  these  changes,  the  more  robust  the  design  of  the  product  is.  In  a  concur- 
rent engineering  environment,  these  design  changes  would  evidently  be  minimal. 

3.  Reduced  product  development  and  design  times  will  result  from  listening  to  the 
voice  of  the  customer  and  from  transferring  information  between  the  various  de- 
partments involved,  including  those  downstream.  This  benefit  is,  in  fact,  a  conse- 
quence of  the  reduction  in  the  number  of  design  iterations  necessary  to  achieve 
optimum  product  design.  Another  factor  is  forsaking  sequential  methods  of  product 
development  and  replacing  them  with  concurrent  ones. 

4.  Reduced  production  cost  is  a  consequence  of  the  preceding  two  benefits — namely, 
the  reduction  in  the  number  of  design  changes  after  releasing  the  product  and  the 
reduction  in  the  time  of  the  product  development  process.  Reduced  cost,  of  course, 
provides  a  manufacturing  company  with  a  real  advantage  in  meeting  global  com- 
petitive pressures. 

5.  Elimination  of  delays  when  releasing  the  product  to  the  market  will  guarantee  a 
good  market  share  for  the  new  product.  Also,  it  has  been  proven  that  delays  in  re- 
leasing the  product  will  result  in  market  loss  of  revenues. 

6.  As  a  result  of  the  reduced  design  time  and  effort,  new  products  will  be  pumped  into 
the  market  more  frequently,  which  is,  indeed,  the  advantage  that  Japanese  au- 
tomakers have  over  their  American  counterparts.  They  can  produce  more  different 
models,  with  smaller  production  volumes  and  shorter  life  cycles. 

7.  Increased  reliability  and  customer  satisfaction  will  result  from  delivering  the  prod- 
uct "right  the  first  time"  and  will  also  enhance  the  credibility  of  the  manufacturing 
company. 


2.3  FACTORS  PREVENTING  THE  ADOPTION 
OF  CONCURRENT  ENGINEERING 


Now,  if  adopting  concurrent  engineering  results  in  all  the  benefits  just  listed,  why  isn't 
it  widely  applied  in  all  manufacturing  corporations?  The  reason  is  that  concurrent  en- 
gineering is  based  on  a  manufacturing  philosophy  that  requires  breaking  barriers  be- 
tween departments  and  establishing  multidisciplinary  teams.  This  philosophy  clearly 
contradicts  the  authoritative  culture  that  is  currently  dominant  in  the  industrial  estab- 
lishment. The  threat  of  loss  of  power  and  authority  makes  middle  management  and  bu- 
reaucrats resistant  to  the  idea  of  implementing  concurrent  engineering.  There  is  also  a 
natural  resistance  to  anything  new  inherent  in  the  minds  of  some  people.  Another  fac- 
tor may  be  the  need  to  build  an  excellent  communication  infrastructure  for  facilitating 
the  flow  of  information  throughout  the  product  development  cycle.  Apparently,  a  lot  of 
money  and  effort  must  be  invested  to  create  an  adequate  information  system — some- 
thing that  many  companies  either  cannot  afford  or  do  not  want  to  do.  Yet  another  fac- 
tor that  holds  up  the  implementation  of  concurrent  engineering  is  the  temptation  to 
come  up  with  temporary  short-run  solutions  to  the  problem  of  decreasing  revenues, 


22  2      Concurrent  Engineering 


without  any  regard  to  strategic  planning  and  long-term  goals.  Examples  include  cutting 
the  work  force  to  increase  profits  (a  faulty,  shortsighted  approach  that  would  cause  a 
company  to  lose  trained  employees  who  are  needed  to  enable  quick,  on-schedule  prod- 
uct delivery)  and  cutting  the  price  without  any  basis  (a  solution  that  would  inevitably 
eliminate  or  reduce  the  profits). 


2.4  THE  FOUR  PILLARS 

OF  CONCURRENT  ENGINEERING 


The  implementation  of  concurrent  engineering  is  based  on  managing  forces  of  change 
and  using  them  as  resources  or  tools  in  four  arenas  for  efficient,  fast,  and  economical 
product  development.  These  arenas — organization,  communication  infrastructure,  re- 
quirements, and  product  development — are  the  pillars  on  which  the  methodology  of 
concurrent  engineering  rests.  Let  us  examine  each  of  them  and  see  how  each  can  be 
managed. 

Organization 

This  arena  includes  the  managers,  product  development  teams,  and  support  teams  (i.e., 
the  organization  itself  and  the  interactions  of  its  components).  The  role  of  management 
is  vital  and  includes  not  only  motivating  people  to  change  their  work  habits  to  match 
the  concurrent  engineering  environment  but  also  ensuring  unhindered  exchange  of  in- 
formation between  the  different  disciplines.  In  fact,  management  can  be  used  as  one  of 
the  forces  of  change  or  tools  that  guarantees  continuous  improvement  of  the  product 
development  process,  as  will  be  discussed  later. 

Communication  Infrastructure 

The  communication  infrastructure  encompasses  the  hardware,  software,  and  expertise 
that  together  form  a  system  that  allows  the  easy  transfer  of  information  relating  to 
product  development.  As  you  may  expect,  when  the  product  complexity  increases,  the 
number  of  disciplines  involved  also  increases,  as  does  the  volume  of  information  to  be 
transferred.  The  system  to  be  established  must  be  capable  of  handling  the  type  and 
amount  of  data  necessary  for  the  product  development  process.  It  must  retrieve,  eval- 
uate, and  present  the  data  in  an  organized  format  that  is  easy  to  understand  and  to  use 
by  team  members  and  by  management.  In  fact,  many  corporations  learned  the  hard 
way  that  communication  technologies  are  as  important  as  design  and  manufacturing 
technologies  for  the  success  of  a  new  product.  The  first  task  to  handle,  after  purchas- 
ing the  hardware,  is  to  build  a  comprehensive  and  efficient  database  that  has  queries 
and  that  can  be  accessed  by  teams  and  by  managers  who  are  in  charge  of  monitoring 
and  evaluating  the  product  development  process.  Electronic  mail,  interactive  browsing 
capabilities,  and  other  modern  information  transfer  technology  are  also  essential  in 
order  to  eliminate  the  need  for  shoveling  piles  of  documents  and  papers  between  the 
different  departments  and  teams. 


2.4  The  Four  Pillars  of  Concurrent  Engineering  23 

Requirements 

A  broad  (but  accurate)  description  of  the  product  requirements  involves  all  product  at- 
tributes that  affect  customer  satisfaction.  Consequently,  customers'  needs  are  considered 
when  setting  the  specifications  for  the  conceptual  design.  This  consideration  would,  in- 
deed, ensure  that  the  model  or  prototype  meets  the  original  goals  from  the  start.  This 
process  of  creating  the  conceptual  specifications  is  extremely  important  and  must  be  car- 
ried out  rigorously.  The  more  product  attributes  and  constraints  are  initially  specified,  the 
fewer  the  problems  associated  with  the  final  product  design  are,  and  the  fewer  the  number 
of  design  changes  or  iterations  will  be.  Of  course,  the  conceptual  design  constraints 
should  be  defined  very  clearly  and  subjected  to  a  continuous  process  of  updating,  evalua- 
tion, and  validation.  As  is  well  known,  constraints  like  government  regulations,  envi- 
ronmental laws,  and  industry  and  national  standards  are  changing  all  the  time. 
Continually  updating  these  would  certainly  improve  the  product  development  process. 


Product  Development 

In  a  concurrent  engineering  environment,  the  downstream  processes  of  manufacturing, 
maintenance,  customer  service,  sales,  and  so  on,  must  be  considered  in  the  early  design 
phase.  This  consideration,  as  previously  mentioned,  is  a  necessary  condition  for  the  im- 
plementation of  concurrent  engineering.  The  second  important  condition  is  the  need  for 
continuous  improvement  and  optimization  of  the  product  development  process.  As  a 
consequence  of  these  two  conditions,  there  is  a  continuous  drive  to  develop,  evaluate, 
and  adopt  new  design  methodologies.  Concepts  and  approaches  like  design  for  manu- 
facturing (DFM)  and  design  for  assembly  (DFA)  have  been  popular  in  recent  days  and 
have  proven  to  be  valuable  tools  in  adopting  and  implementing  concurrent  engineering. 
The  reason  is  that  their  philosophy  is  based  on  using  manufacturing  (or  assembly)  as  a 
design  constraint,  thus  taking  downstream  processes  like  manufacturing  and  assembly 
into  full  consideration  during  the  early  design  phase  of  the  product.  In  other  words,  the 
success  of  these  methodologies  is  linked  to  their  philosophies  being  compatible  to  (or 
matching)  that  of  concurrent  engineering. 

Another  part  of  the  product  development  arena  is  what  is  sometimes  referred  to  as 
the  component  libraries.  The  design  (and  manufacturing)  attributes  of  the  different 
components,  whether  standard  ones  that  were  purchased  or  parts  that  were  previously 
designed  and  manufactured,  are  kept  in  a  database.  The  availability  of  such  a  database 
to  team  members  will  speed  up  the  design  process  by  providing  them  with  many  al- 
ternatives to  choose  from  and,  more  importantly,  by  freeing  them  from  reinventing  the 
wheel.  Using  previously  tested  components,  maybe  with  very  slight  modification  in 
the  design,  can  save  a  lot  of  time  and  effort.  Keeping  a  computerized  database  has  the 
advantages  of  easy  retrieval  of  designs  and  simultaneous  availability  to  all  team  mem- 
bers. This  topic  will  be  covered  later  in  the  book  in  detail  when  we  discuss  group  tech- 
nology and  computer-aided  process  planning. 

Also  part  of  the  product  development  arena  is  the  design  process  itself.  We  have 
already  covered  its  stages  and  methodology  in  Chapter  1.  Here,  we  want  to  emphasize 
again  that  good  design  has  always  been  based  on  customer  needs,  which  must  be,  in 


24 


Concurrent  Engineering 


turn,  determined  by  listening  to  customer  concerns.  In  fact,  it  was  for  this  reason  that 
the  concept  of  quality  function  deployment  (QFD)  was  developed  in  Japan's  Kyoto 
Shipyard  in  the  1980s.  By  including  QFD  in  the  design  process,  teams  do  not  lose 
touch  with  the  customer,  and,  consequently,  the  designed  products  will  meet  cus- 
tomers' needs  and  expectations.  Although  QFD  is  beyond  the  scope  of  this  text,  a  brief 
discussion  will  enlighten  those  engineers  who  must  communicate  with  members  in 
charge  of  QFD  in  a  multidisciplinary  team.  QFD  seeks  to  identify  and  evaluate  the 
meaning  of  the  word  quality  from  the  customer's  point  of  view.  The  approach  involves 
constructing  a  matrix  that  is  quite  similar  to  the  design  decision  matrix  covered  in 
Chapter  1  (see  Figure  1.9).  This  matrix  is  called  the  house  of  quality.  The  attributes, 
functions,  and  characteristics  that  the  customer  wants  can  be  clearly  identified  and 
used  as  input  constraints  or  requirements  for  the  process  of  designing  the  product  as 
previously  mentioned.  To  repeat,  remember  that  one  of  the  goals  of  the  design  team  is 
to  have  a  decreasing  number  of  design  changes  with  increasing  order  of  design  stage. 
In  the  final  design  stage,  if  the  appropriate  methodology  is  adopted,  the  number  of 
changes  should  be  zero. 


ORCES  OF  CHANGE 


Implementing  concurrent  engineering  is  based  on  managing  some  forces  of  change  and 
using  them  as  resources  or  tools  to  create  the  concurrent  engineering  environment.  Fol- 
lowing are  some  of  these  forces  of  change. 


Technology 

Technology  has  a  very  important  role  to  play  in  each  of  the  four  arenas  of  concurrent 
engineering.  It  speeds  up  and  optimizes  the  product  development  process,  minimizes 
the  number  of  design  iterations,  and  facilitates  communication  and  information  trans- 
fer between  the  different  teams  and  departments.  Managers  should,  therefore,  take  full 
advantage  of  the  most  up-to-date  technology  available  and  avoid  technology  that  is 
under  development  or  obsolete.  Unfortunately,  the  problem  of  acquiring  up-to-date 
technology  is  far  more  complicated  than  it  seems  because  of  the  extremely  fast  pace 
with  which  technology  is  advancing  and  the  vast  amount  of  different  options  available. 
Here  are  some  tips  that  address  the  technology  problem: 

1.  Keep  engineers  abreast  of  the  latest  technological  developments  by  providing  them 
with  technical  journals  and  periodicals,  sending  them  to  international  engineering 
conferences  and  exhibitions,  and  ensuring  a  continuous  learning  process  through 
workshops  and  short  courses  offered  on  site. 

2.  Ensure  that  the  latest  scientific  findings  are  promptly  employed  in  developing  a  com- 
pany's technology  and,  therefore,  result  in  high-quality  products.  Companies  should 
focus  their  efforts  on  applied  research  for  developing  products  and  processes  and  in- 
tegrate their  R&D  with  design  and  development  activities.  (In  fact,  this  is  one  of  the 
reasons  behind  the  success  of  several  countries  in  the  Far  East.) 


2.5  Forces  of  Change  25 

3.  Try  to  make  use  of  the  results  of  government-funded  research  and  thus  save  time 
and  money  spent  in  obtaining  similar  findings. 

4.  Definitely  overcome  the  "not-invented-here"  syndrome.  Many  industry  people 
make  the  mistake  of  completely  ignoring  any  technology  that  was  not  invented  in 
their  company.  This  syndrome  leads  to  isolationism  and,  eventually,  falling  behind. 
It  is  very  difficult  for  a  company  to  fully  develop  technology  starting  from  scratch. 
Acquiring  technology  by  purchasing  it,  by  establishing  partnerships  between  com- 
panies, and  by  encouraging  technology  exchanges  is  worth  exploring. 

It  is  important  here  to  cast  light  on  one  of  industry's  most  difficult  problems  in  the 
United  States — the  bad  effects  of  having  advanced  technology  geared  toward  military 
applications.  Although  there  is  a  wealth  of  technological  information  as  a  result  of  ac- 
tive R  &  D  in  military  industries,  it  is  classified  and,  therefore,  not  accessible  for  civilian  in- 
dustries and  commercial  applications.  A  further  obstacle  is  the  difference  in  the  product 
requirements  in  both  cases.  Although  military  criteria  specify  quality  regardless  of  cost, 
civilian  requirements  call  for  both  quality  and  cost.  The  picture  is  clear  when  you  compare 
the  performance  and  the  cost  of  a  nuclear  bomber  with  those  of  a  commercial  jetliner. 

Management 

Management  in  a  concurrent  engineering  environment  takes  its  role  from  management 
in  a  traditional  serial  manufacturing  corporation  and  goes  far  beyond  it.  A  manager's 
role  involves  not  only  setting  schedules  and  work  expectations  of  engineers  and  assign- 
ing responsibilities  but  also  managing  changes  and  building  an  organizational  structure 
that  is  flexible  and  can  respond  quickly  to  surprises  and  sudden  changes  in  demands  and 
requirements.  You  may  have  already  concluded  that  managers  must  have  a  general  but 
solid  understanding  of  current  and  relevant  technical  issues  in  order  to  communicate  ef- 
fectively with  multidisciplinary  teams.  In  fact,  one  of  the  most  important  tasks  of  mid- 
dle management  in  a  concurrent  engineering  environment  is  the  creation  of  those 
multidisciplinary  teams  in  order  to  carry  out  the  product  development  process.  In  sum- 
mary, the  traditional  role  of  management  that  is  based  on  vertical  chain  of  command,  au- 
thoritative decision  making,  and  the  "carrot-stick"  model  of  running  corporations  is 
diminishing  continuously,  especially  in  a  company  that  adopts  the  concurrent  engineer- 
ing philosophy.  More  emphasis  is  being  placed  on  creating  product  development  teams 
and  facilitating  information  transfer  and  communication  between  them. 

Let  us  look  more  thoroughly  at  the  process  of  establishing  multidisciplinary  teams. 
As  you  may  expect,  complex  tasks  are  handled  by  breaking  them  into  less  complex  ones 
that  are,  in  turn,  dealt  with  simultaneously  but  separately  with  different  teams.  Good  man- 
agers should  optimize  the  size  of  each  team.  A  team  that  is  too  large  or  too  small  creates 
communication  problems,  is  less  efficient,  and  is  more  expensive.  Attention  must  also  be 
given  to  the  talents  and  the  quality  of  team  members  in  terms  of  choosing  the  right  person 
for  the  right  job.  When  establishing  the  teams,  the  management  focus  must  be  to  concur- 
rently execute  tasks  that  are  normally  carried  out  sequentially  and  to  integrate  those  ac- 
tivities that  are  concurrent.  Consequently,  an  appropriate  project-modeling  tool  must  be 
used  in  order  to  identify  and  locate  the  patterns  of  information  flow  and  interaction.  There 


26 


Concurrent  Engineering 


FIGURE    2.1 

The  PERT  chart 


are  basically  three  approaches  or  tools — namely,  the  PERT  chart,  the  GANTT  chart,  and 
the  design  structure  matrix  (DSM). 

The  PERT  chart,  which  is  illustrated  in  Figure  2.1,  is  basically  used  to  determine 
project  duration  and  critical  path.  On  the  other  hand,  the  GANTT  chart  displays  the 
relative  positioning  of  tasks  on  a  time  scale,  as  shown  in  Figure  2.2.  In  fact,  some  re- 
searchers believe  that  the  DSM  method  is  far  better  in  displaying  the  connectivity  of 
interacting  tasks  and  improving  the  product  development  process.  It  also  clearly  illus- 
trates where  the  integration  of  tasks  should  take  place. 

The  DSM  method  and  its  modified  version  have  been  extensively  used  by  Smith 
and  Eppinger  of  the  Sloan  School  of  Management  at  the  Massachusetts  Institute  of 
Technology  (MIT).  The  basic  method  involves  representing  the  relationship  among 
project  tasks  in  a  matrix  form  and  allows  for  different  tasks  to  be  coupled.  As  can  be 
seen  in  Figure  2.3,  each  individual  task  is  represented  by  a  row  and  by  a  column  of  a 
square  matrix;  the  need  for  information  flow  between  two  tasks  is  indicated  by  a  check 
mark  (x).  Going  horizontally  across  a  task's  row,  the  columns  under  which  there  are 
check  marks  are  those  from  which  information  must  be  received  in  order  to  complete 
the  given  task.  On  the  other  hand,  going  vertically  down  a  task's  column,  the  check 


FIGURE    2.2 

The  GANTT  chart 


Task  A 

/       ' 

' 

TaskB 

' 

' 

TaskC 

' 

'/ 

Task  D 

2.5  Forces  of  Change 


27 


FIGURE    2.3 

Initial  phase  of  the 
design  structure  matrix 


Tasks  ABCDEFGH      I 
A 
B 
C 

D 

E 
F 
G 
H 


KLMNOPQRSTUV 


A 

X 

X 

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marks  indicate  the  rows  (tasks)  that  require  output  from  the  given  column.  The  diago- 
nal elements  are  hatched  because  a  task  cannot  be  coupled  with  itself.  Now,  structur- 
ing the  teams,  usually  referred  to  as  product  development  teams  (PDTs),  can  be 
accomplished  by  identifying  highly  coupled  sets  of  tasks.  First,  the  rows  and  columns 
must  be  rearranged  so  as  to  yield  "batches"  of  check  marks  where  a  few  tasks  are  cou- 
pled together  and  where  the  information  of  PDTs  is  most  appropriate,  as  shown  in  Fig- 
ure 2.4.  The  process  of  swapping  the  rows  and  columns  of  the  matrix  requires 
experience  and  skill  because  it  is  based  on  trial  and  error.  Nevertheless,  the  process  is 
also  amenable  to  computer  manipulation  and  analysis.  As  you  can  see,  however,  the 
DSM  model  does  not  take  into  consideration  the  degree  of  dependence  or  coupling  be- 
tween each  two  tasks.  Recently,  Smith  and  Eppinger  replaced  the  check  marks  by 
numbers  indicating  the  strength  of  dependence.  The  eigenvalue  of  such  a  matrix  would 
reveal  the  highly  coupled  sets  of  tasks. 

After  the  teams  are  established,  the  next  question  is  how  to  manage  the  product 
development  project  and  ensure  that  it  is  on  target  to  meet  the  previously  agreed-upon 
milestones  and  deadlines.  Again,  PERT  and  GANTT  charts  can  be  employed  to 


28 


Concurrent  Engineering 


FIGURE    2.4 

Final  phase  of  the 
design  structure  matrix 


Tasks  A  F  G  D  E 
A 
F 
G 
D 
E 
I 

B 
C 
J 
K 
P 
H 
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BCJ      KPHNOQLMRSTUV 


A 

X 

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achieve  these  goals.  There  are,  however,  some  other  methods  for  project  updating  that 
visually  illustrate  the  project  status  in  one  integrated  chart,  thus  ensuring  that  different 
PDTs  meet  their  stated  goals  concurrently.  The  radar  (spider)  chart  is  a  popular  one.  As 
can  be  seen  in  Figure  2.5,  each  task  or  area  of  activity  is  represented  by  a  radial  line 
and  for  a  one-year  period.  Thus,  one  look  at  the  chart  is  enough  to  see  whether  or  not 
the  separate  goals  in  the  different  areas  are  met.  In  the  ideal  case,  when  all  tasks  are 
performed  exactly  according  to  the  planned  schedule,  this  chart  will  end  up  having 
concentric  circles  at  the  various  time  periods,  as  indicated  by  the  dashed  lines  in  Fig- 
ure 2.5.  On  the  other  hand,  the  bug  chart  is  a  plot  of  project  expenditures  versus  prod- 
uct goals  or  milestones,  which  are  indicated  on  the  time  axis  as  shown  in  Figure  2.6. 
The  scales  of  both  axes  are  adjusted  so  that  a  project  that  is  on  track  is  represented  by 
a  straight  line  making  45°  with  both  axes.  Although  this  chart  has  the  advantages  of  in- 
dicating project-cost  updating  and  individual  milestones,  it  is  sometimes  misleading  (a 
delay  in  purchasing  supplies,  for  example,  might  seem  or  be  interpreted  as  a  positive 
indication).  Further  details  are  beyond  the  scope  of  this  text,  and  interested  readers  are 
advised  and  encouraged  to  seek  specialized  books  on  the  subject. 


2.5  Forces  of  Change 


29 


FIGURE    2.5 

The  radar  (spider)  chart 


Area  of  activity 

1 


FIGURE    2.6 

The  bug  chart 


Manufacturing 
release 
exenditure 
target 


Actual 

manufacturing 
release  date 
and  expenditure 


Milestone 


Manufacturing  release 
date  target 


30  2      Concurrent  Engineering 


Tools 


There  is  an  extremely  large  number  of  tools  for  handling  various  tasks  in  the  different 
arenas.  The  selection  of  the  right  tool  for  the  right  job  is,  therefore,  not  easy.  In  addi- 
tion, most  of  these  tools  are  undergoing  a  rapid,  continuous,  never-ending  evolution. 
Tools  that  were  new  in  the  1980s  are  now  technically  obsolete,  which  indicates  the 
need  to  continually  upgrade  and  replace  a  company's  acquired  tools.  For  example, 
structured  analysis  software  tools  manage  information  systems  and  present  complex 
systems  in  a  clear,  easy-to-comprehend  way.  Instead  of  the  old-fashioned  written  flow- 
charting procedures,  this  software  provides  graphical  representation  of  any  complex 
operation  by  a  network  of  elements  (each  stands  for  a  particular  function)  and  by  ar- 
rows indicating  data  flow  and  interaction  between  those  elements.  It  also  enables  the 
elimination  of  redundant  loops,  thus  making  an  operation  more  efficient. 

Another  example  is  tools  used  for  design  automation.  They  find  increasing  appli- 
cation in  manufacturing  corporations,  and  it  is  anticipated  that  by  the  year  2000  about 
80  percent  of  all  designs  will  be  electronically  done  using  these  tools.  Also,  integration 
of  these  islands  of  automation  (i.e.,  engineering  departments  in  a  manufacturing  firm) 
is  the  trend  in  the  1990s,  where  local-area  networks  (LANs)  are  extensively  used  to 
transfer  information  from  one  department  to  another  in  a  standardized  format. 

The  adoption  of  new  tools  creates  the  problem  of  changing  the  responsibilities  and 
nature  of  the  jobs  of  employees,  who  will  need  retraining  and,  sometimes,  have  to  be 
swapped.  Also,  based  on  the  preceding  discussion,  the  level  of  automation  used  must 
be  appropriate  for  the  company,  and  automated  PDTs  must  be  integrated  into  a  system. 

It  is  always  important  to  remember  that  the  forces  of  change  discussed  herein  are 
just  examples  and  that  there  can  be  other  forces  of  change  depending  upon  the  nature  of 
the  manufacturing  corporation  and  its  production.  Nevertheless,  in  all  cases  and  regard- 
less of  the  tools  used,  the  change  from  serial  manufacturing  to  concurrent  engineering 
must  be  well  planned  and  managed  so  as  to  take  place  gradually  and  smoothly  and  must 
always  be  monitored  by  management.  In  fact,  abrupt  changes  and  employee  dissatisfac- 
tion are  two  factors  that  can  impede  the  implementation  of  concurrent  engineering. 


>   ~ 


J.6  A  SUCCESS  STORY:  NIPPONDENSO 

Now  that  you  understand  the  arenas  of  concurrent  engineering  and  the  forces  of 
change,  it  is  time  to  look  at  a  case  study  indicating  how  concurrent  engineering  was 
successfully  implemented  and  resulted  in  solving  tough  problems  that  were  facing  one 
of  the  world's  largest  manufacturers  of  automotive  parts.  The  original  report  was  given 
in  a  paper  entitled  "Nippondenso  Co.  Ltd:  A  Case  Study  of  Strategic  Product  Design," 
authored  by  Daniel  E.  Whitney  and  presented  at  the  Collaborative  Engineering  Con- 
ference held  at  MIT  in  October  1993.  This  paper  contained  a  wealth  of  information 
and  was  based  on  seven  personal  visits  by  the  author  to  Nippondenso  Co.  Ltd.  during 
the  period  1974  to  1991,  as  well  as  on  interviews  with  the  company's  personnel  and 
papers  published  by  the  engineering  staff.  The  information  has  been  rearranged  here, 
however,  so  as  to  draw  parallelism  with  the  previously  mentioned  concurrent  engi- 
neering model  and  its  four  arenas. 


2.6  A  Success  Story:  Nippondenso  31 

Nippondenso  Co.  Ltd.  is  one  of  the  world's  largest  manufacturers  of  automotive 
components,  including  air  conditioners,  heaters,  relays,  alternators,  radiators,  plus  me- 
ters, diesel  components,  filters,  controls,  brake  systems,  and  entertainment  equipment. 
The  company  has  20  plants  in  15  foreign  countries  in  addition  to  10  plants  in  Japan.  In 
1991,  almost  43,000  people  were  employed  by  the  company  worldwide.  Nippondenso 
is  the  first-tier  supplier  to  Toyota  and  other  Japanese  and  foreign  car  companies,  and 
its  sales  amounted  to  about  $10  billion  dollars  in  1989.  Now  that  you  have  a  clear  idea 
about  the  size  of  this  company  and  the  diversity  of  its  products,  let  us  see  how  they 
created  a  concurrent  engineering  environment.  Following  is  the  company's  approach 
in  each  of  the  previously  mentioned  arenas. 

Organization 

Nippondenso's  philosophy  is  based  on  developing  the  product  and  the  process  for  mak- 
ing it  simultaneously.  Consequently,  multidisciplinary  teams  are  formed  through  repre- 
sentation from  various  departments  like  production  engineering,  machines  and  tools, 
product  design,  and  so  on.  Teams  are  small  at  the  beginning  but  become  larger  as  the 
project  proceeds  from  the  concept  phase  to  the  detailed-design  phase.  Top  management 
promptly  steps  in  when  a  crisis  occurs  and  when  a  crucial  decision  needs  to  be  made.  Of 
course,  a  parallel-task  approach  is  employed  by  overlapping  some  of  the  design  steps. 

Requirements 

In  addition  to  product  performance  specifications  and  production  cost  targets,  there  are 
other  severe  constraints  dictated  by  the  nature  of  the  business  of  Nippondenso  as  a 
supplier  to  large  auto  manufacturers  (i.e.,  the  need  to  meet  ordering  patterns).  The  re- 
quirements of  customers  (like  Toyota)  include  delivering  extremely  large  amounts  of 
products  on  a  just-in-time  (JIT)  basis,  with  high  variety  and  an  unpredictable  model 
mix  that  is  always  changing.  A  further  constraint  is  to  achieve  all  these  goals  with  lit- 
tle or  no  changeover  time.  As  you  will  see  later,  defining  customer  requirements 
helped  Nippondenso  to  address  the  problems  in  a  rational,  thoughtful  manner. 

Communication  Infrastructure 

Nippondenso  built  an  excellent  system  for  information  exchange.  It  is  used  to  integrate 
the  different  machine  tools  throughout  the  plant  through  local-  or  wide-area  networks 
that  are,  in  turn,  linked  with  the  engineering  departments  dealing  with  computer  con- 
trol, scheduling,  quality  monitoring,  and  the  like.  Any  change  in  data  by  a  team  mem- 
ber is  promptly  made  available  to  all  members  of  other  teams,  thus  breaking  the 
barriers  between  departments  and  between  teams. 

Product  Development 

The  two  most  important  elements  upon  which  Nippondenso's  approach  in  the  product 
development  arena  is  based  include  developing  the  product  and  its  manufacturing 
processes  simultaneously  and  developing  new  product  design  methodologies.  In  fact, 
this  approach  is  credited  for  enabling  Nippondenso  to  meet  customer  requirements. 


32 


Concurrent  Engineering 


In  order  to  meet  the  challenge  of  high  production  volume  and  high  variety,  the 
first  step  for  Nippondenso  was  standardization  after  negotiating  with  customers  and 
listening  to  their  concerns.  The  next  step  was  to  design  the  products  intelligently  so  as 
to  achieve  the  desired  flexibility  during  assembly,  rather  than  employing  complex  and 
expensive  production  methods.  In  other  words,  their  philosophy  was  based  on  using 
assembly  rather  than  manufacturing  to  make  different  models.  High  variety  was 
achieved  by  producing  several  versions  of  each  component  in  the  product  and  then  as- 
sembling the  appropriate  component's  versions  into  any  desired  model.  Thus,  an  ex- 
tremely large  number  of  combinations  of  component  versions  resulted  in  a  large 
number  of  possible  models.  Moreover,  this  approach  also  ensured  quick  changeover 
from  one  model  to  another. 

At  this  point,  the  basic  concept  of  concurrent  engineering  has  been  thoroughly 
demonstrated.  Interested  readers  are  encouraged  to  consult  more  specialized  books  on 
the  subject  (see  the  titles  provided  in  the  references  at  the  back  of  the  book). 


>w  Questions 


1.  Define  the  term  concurrent  engineering  and 
elaborate  on  its  meaning. 

2.  In  what  way  does  a  concurrent  engineering  en- 
vironment differ  from  that  of  serial  manufactur- 
ing? 

3.  How  did  concurrent  engineering  come  into 
being? 

4.  What  are  the  reasons  for  adopting  concurrent 
engineering? 

5.  Discuss  three  of  these  reasons  in  detail. 

6.  List  the  benefits  of  adopting  concurrent  engi- 
neering and  discuss  three  of  them  in  detail. 

7.  If  concurrent  engineering  is  so  beneficial,  why 
don't  all  manufacturing  companies  adopt  it? 


10. 


11. 


What  are  the  four  pillars  on  which  concurrent 
engineering  rests? 

What  is  the  difference  between  the  role  of  man- 
agement in  a  concurrent  engineering  environ- 
ment and  that  role  in  conventional  serial 
manufacturing? 

How  does  the  product  development  process  dif- 
fer in  a  concurrent  engineering  environment 
from  that  in  conventional  serial  manufacturing? 

Explain  why  new  concepts  like  DFM,  DFA, 
and  QFD  are  important  and  very  useful  when 
implementing  concurrent  engineering. 


INTRODUCTION 

Definition.  The  word  casting  is  used  both  for  the  process  and  for  the  product. 
The  process  of  casting  is  the  manufacture  of  metallic  objects  (castings)  by 
melting  the  metal,  pouring  it  into  a  mold  cavity,  and  allowing  the  molten  metal 
to  solidify  as  a  casting  whose  shape  is  a  reproduction  of  the  mold  cavity.  This 
process  is  carried  out  in  a  foundry,  where  either  ferrous  (i.e.,  iron-base)  or  non- 
ferrous  metals  are  cast. 

Casting  processes  have  found  widespread  application,  and  the  foundry  in- 
dustry is  considered  to  be  the  sixth  largest  in  the  United  States  because  it  pro- 
duces hundreds  of  intricately  shaped  parts  of  various  sizes  like  plumbing 
fixtures,  furnace  parts,  cylinder  blocks  of  automobile  and  airplane  engines,  pis- 
tons, piston  rings,  machine  tool  beds  and  frames,  wheels,  and  crankshafts.  In 
fact,  the  foundry  industry  includes  a  variety  of  casting  processes  that  can  be 
classified  in  one  of  the  following  three  ways: 

1.  By  the  mold  material  and/or  procedure  of  mold  production 

2.  By  the  method  of  filling  the  mold 

3.  By  the  metal  of  the  casting  itself 

Historical  Background.  At  the  dawn  of  the  metal  age,  human  knowledge  was  not 
advanced  enough  to  generate  the  high  temperatures  necessary  for  smelting 
metals.  Therefore,  because  casting  was  not  possible,  metals  were  used  as 
found  or  heated  to  a  soft  state  and  worked  into  shapes.  The  products  of  that  era 
are  exemplified  by  the  copper  pendant  from  Shanidar  Cave  (northeast  of  Iraq), 
which  dates  back  to  9500  b.c.  and  which  was  shaped  by  hammering  a  piece  of 


33 


34  3      Casting  and  Foundry  Work 


native  metal  and  finishing  with  abrasives.  Later,  copper-smelting  techniques 
were  developed,  and  copper  castings  were  produced  in  Mesopotamia  as  early  as 
3000  b.c.  The  art  of  casting  was  then  refined  by  the  ancient  Egyptians,  who  in- 
novated the  "lost-wax"  molding  process.  During  the  Bronze  Age,  foundry  work 
flourished  in  China,  where  high-quality  castings  with  intricate  shapes  could  be 
produced.  The  Chinese  developed  certain  bronze  alloys  and  mastered  the  lost- 
wax  process  during  the  Shang  dynasty.  Later,  that  art  found  its  way  to  Japan  with 
the  introduction  of  Buddhism  in  the  sixth  century.  There  were  also  some  signifi- 
cant achievements  in  the  West,  where  the  Colossus  of  Rhodes,  a  statue  of  the 
sun  god  Helios  weighing  360  tons,  was  considered  to  be  one  of  the  seven  won- 
ders of  the  world.  That  bronze  statue  was  cast  in  sections,  which  were  assem- 
bled later,  and  stood  105  feet  high  at  the  entrance  of  the  harbor  of  Rhodes. 

Although  iron  was  known  in  Egypt  as  early  as  4000  b.c,  the  development  of 
cast  iron  was  impossible  because  the  high  melting  temperature  needed  was  not 
achievable  then  and  pottery  vessels  capable  of  containing  molten  iron  were  not 
available.  The  age  of  cast  iron  finally  arrived  in  1340  when  a  flow  oven  (a  crude 
version  of  the  blast  furnace)  was  erected  at  Marche-Les-Dames  in  Belgium.  It  was 
capable  of  continuous  volume  production  of  molten  iron.  Ferrous  foundry  practice 
developed  further  with  the  invention  of  the  cupola  furnace  by  John  Wilkenson  in 
England.  This  was  followed  by  the  production  of  black-heart  malleable  iron  in 
1826  by  Seth  Boyden  and  the  development  of  metallography  by  Henry  Sorby  of 
England.  The  relationship  between  the  properties  and  the  microstructure  of  alloys 
became  understood,  and  complete  control  of  the  casting  process  became  feasi- 
ble based  on  this  knowledge.  Nevertheless,  forming  processes  developed  more 
rapidly  than  foundry  practice  because  wrought  alloys  could  better  meet  a  wider 
range  of  applications.  Nodular  cast  iron,  which  possesses  both  the  castability  of 
cast  iron  and  the  impact  strength  of  steel,  was  introduced  in  1948,  thus  paving 
the  way  for  castings  to  compete  more  favorably  with  wrought  alloys. 


3.1  CLASSIFICATIONS  OF  CASTING 
BY  MOLD  MATERIAL 


Molds  can  be  either  permanent  or  nonpermanent.  Permanent  molds  are  made  of  steel, 
cast  iron,  and  even  graphite.  They  allow  large  numbers  of  castings  to  be  produced  suc- 
cessively without  changing  the  mold.  A  nonpermanent  mold  is  used  for  one  pouring 


3.1  Classifications  of  Casting  by  Mold  Material  35 

only.  It  is  usually  made  of  a  silica  sand  mixture  but  sometimes  of  other  refractory  ma- 
terials like  chromite  and  magnesite. 

Green  Sand  Molds 

Molding  materials.  Natural  deposits  taken  from  water  or  riverbeds  are  used  as  mold- 
ing materials  for  low-melting-point  alloys.  Thus,  the  material  is  called  green  sand, 
meaning  unbaked  or  used  as  found.  These  deposits  have  the  advantages  of  availability 
and  low  cost,  and  they  provide  smooth  as-cast  surfaces,  especially  for  light,  thin  jobs. 
However,  they  contain  15  to  25  percent  clay,  which,  in  turn,  includes  some  organic  im- 
purities that  markedly  reduce  the  fusion  temperatures  of  the  natural  sand  mixture, 
lower  the  initial  binding  strength,  and  require  a  high  moisture  content  (6  to  8  percent). 
Therefore,  synthetic  molding  sand  has  been  developed  by  mixing  a  cleaned  pure  silica 
sand  base,  in  which  grain  structure  and  grain-size  distribution  are  controlled,  with  up 
to  18  percent  combined  fireclay  and  bentonite  and  only  about  3  percent  moisture.  Be- 
cause the  amount  of  clay  used  as  a  binding  material  is  minimal,  synthetic  molding  sand 
has  higher  refractoriness,  higher  green  (unbaked)  strength,  better  permeability,  and 
lower  moisture  content.  The  latter  advantage  results  in  the  evolution  of  less  steam  dur- 
ing the  casting  process.  Thus,  control  of  the  properties  of  the  sand  mixture  is  an  im- 
portant condition  for  obtaining  good  castings.  For  this  reason,  a  sand  laboratory  is 
usually  attached  to  the  foundry  to  determine  the  properties  of  molding  sands  prior  to 
casting.  Following  are  some  important  properties  of  a  green  sand  mixture: 

1.  Permeability.  Permeability  is  the  most  important  property  of  the  molding  sand  and 
can  be  defined  as  the  ability  of  the  molding  sand  to  allow  gases  to  pass  through. 
This  property  depends  not  only  on  the  shape  and  size  of  the  particles  of  the  sand 
base  but  also  on  the  amount  of  the  clay  binding  material  present  in  the  mixture  and 
on  the  moisture  content.  The  permeability  of  molds  is  usually  low  when  casting 
gray  cast  iron  and  high  when  casting  steel. 

2.  Green  compression  strength  of  a  sand  mold.  Green  strength  is  mainly  due  to  the 
clay  (or  bentonite)  and  the  moisture  content,  which  both  bind  the  sand  particles  to- 
gether. Molds  must  be  strong  enough  not  to  collapse  during  handling  and  transfer 
and  must  also  be  capable  of  withstanding  pressure  and  erosion  forces  during  pour- 
ing of  the  molten  metal. 

3.  Moisture  content.  Moisture  content  is  expressed  as  a  percentage  and  is  important 
because  it  affects  other  properties,  such  as  the  permeability  and  green  strength.  Ex- 
cessive moisture  content  can  result  in  entrapped  steam  bubbles  in  the  casting. 

4.  Flowability.  Flowability  is  the  ability  of  sand  to  flow  easily  and  fill  the  recesses  and 
the  fine  details  in  the  pattern. 

5.  Refractoriness.  Refractoriness  is  the  resistance  of  the  molding  sand  to  elevated 
temperatures;  that  is,  the  sand  particles  must  not  melt,  soften,  or  sinter  when  they 
come  in  contact  with  the  molten  metal  during  the  casting  process.  Molding  sands 
with  poor  refractoriness  may  burn  when  the  molten  metal  is  poured  into  the  mold. 
Usually,  sand  molds  should  be  able  to  withstand  up  to  3000°F  (1650°C). 


36  3      Casting  and  Foundry  Work 


Sand  molding  tools.  Sand  molds  are  made  in  flasks,  which  are  bottomless  containers. 
The  function  of  a  flask  is  to  hold  and  reinforce  the  sand  mold  to  allow  handling  and 
manipulation.  A  flask  can  be  made  of  wood,  sheet  steel,  or  aluminum  and  consists  of 
two  parts:  an  upper  half  called  the  cope  and  a  lower  half  called  the  drag.  The  standard 
flask  is  rectangular,  although  special  shapes  are  also  in  use.  For  proper  alignment  of 
the  two  halves  of  the  mold  cavity  when  putting  the  cope  onto  the  drag  prior  to  casting, 
flasks  are  usually  fitted  with  guide  pins.  When  the  required  casting  is  high,  a  middle 
part,  called  the  cheek,  is  added  between  the  drag  and  the  cope.  Also,  when  a  large 
product  is  to  be  cast,  a  pit  in  the  ground  is  substituted  for  the  drag;  the  process  is  then 
referred  to  as  pit  molding. 

Other  sand  molding  tools  can  be  divided  into  two  main  groups: 

1.  Tools  (such  as  molders,  sand  shovels,  bench  rammers,  and  the  like)  used  for  fill- 
ing the  flask  and  ramming  the  sand 

2.  Tools  (such  as  draw  screws,  draw  spikes,  trowels,  slicks,  spoons,  and  lifters)  used 
for  releasing  and  withdrawing  the  pattern  from  the  mold  and  for  making  required 
repairs  on  or  putting  finishing  touches  to  the  mold  surfaces 

Patterns  for  sand  molding.  The  mold  cavity  is  the  impression  of  a  pattern,  which  is 
an  approximate  replica  of  the  exterior  of  the  desired  casting.  Permanent  patterns 
(which  are  usually  used  with  sand  molding)  can  be  made  of  softwood  like  pine,  hard- 
wood like  mahogany,  plastics,  or  metals  like  aluminum,  cast  iron,  or  steel.  They  are 
made  in  special  shops  called  pattern  shops.  Wood  patterns  must  be  made  of  dried  or 
seasoned  wood  containing  less  than  10  percent  moisture  to  avoid  warping  and  dis- 
tortion of  the  pattern  if  the  wood  dries  out.  They  should  not  absorb  any  moisture 
from  the  green  molding  sand.  Thus,  the  surfaces  of  these  patterns  are  painted  and 
coated  with  a  waterproof  varnish.  A  single-piece  wood  pattern  can  be  used  for  mak- 
ing 20  to  30  molds,  a  plastic  pattern  can  be  used  for  20,000  molds,  and  a  metal  pat- 
tern can  be  used  for  up  to  100,000  molds,  depending  upon  the  metal  of  the  pattern. 
In  fact,  several  types  of  permanent  patterns  are  used  in  foundries.  They  include  the 
following: 

1.  Single  or  loose  pattern.  This  pattern  is  actually  a  single  copy  of  the  desired  cast- 
ing. Loose  patterns  are  usually  used  when  only  a  few  castings  are  required  or  when 
prototype  castings  are  produced. 

2.  Gated  patterns.  These  are  patterns  with  gates  in  a  runner  system.  They  are  used  to 
eliminate  the  hand-cutting  of  gates. 

3.  Match-plate  patterns.  Such  patterns  are  used  for  large-quantity  production  of 
smaller  castings,  where  machine  molding  is  usually  employed.  The  two  halves  of 
the  pattern,  with  the  line  of  separation  conforming  to  the  parting  line,  are  perma- 
nently mounted  on  opposite  sides  of  a  wood  or  metal  plate.  This  type  of  pattern  al- 
ways incorporates  the  gating  system  as  a  part  of  the  pattern. 

4.  Cope-and-drag  pattern  plates.  The  function  of  this  type  of  pattern  is  similar  to  that 
of  the  match-plate  patterns.  Such  a  pattern  consists  of  the  cope  and  drag  parts  of  the 


3.1  Classifications  of  Casting  by  Mold  Material  37 

pattern  mounted  on  separate  plates.  It  is  particularly  advantageous  for  preparing 
molds  for  large  and  medium  castings,  where  the  cope  and  drag  parts  of  the  mold  are 
prepared  on  different  molding  machines.  Therefore,  accurate  alignment  of  the  two 
halves  of  the  mold  is  necessary  and  is  achieved  through  the  use  of  guide  and  locat- 
ing pins  and  bushings  in  the  flasks. 

In  order  for  a  pattern  to  be  successfully  employed  in  producing  a  casting  having 
the  desired  dimensions,  it  must  not  be  an  exact  replica  of  the  part  to  be  cast.  A  number 
of  allowances  must  be  made  on  the  dimensions  of  the  pattern: 

1.  Pattern  drafts.  This  is  a  taper  of  about  1  percent  that  is  added  to  all  surfaces  per- 
pendicular to  the  parting  line  in  order  to  facilitate  removal  of  the  pattern  from  the 
mold  without  ruining  the  surfaces  of  the  cavity.  Higher  values  of  pattern  draft  are 
employed  in  the  case  of  pockets  or  deep  cavities. 

2.  Shrinkage  allowance.  Because  molten  metals  shrink  during  solidification  and  con- 
tract with  further  cooling  to  room  temperature,  linear  dimensions  of  patterns  must 
be  made  larger  to  compensate  for  that  shrinkage  and  contraction.  The  value  of  the 
shrinkage  allowance  depends  upon  the  metal  to  be  cast  and,  to  some  extent,  on 
the  nature  of  the  casting.  The  shrinkage  allowance  is  usually  taken  as  1  percent  for 
cast  iron,  2  percent  for  steel,  1.5  percent  for  aluminum,  1.5  percent  for  magnesium, 
1 .6  percent  for  brass,  and  2  percent  for  bronze.  In  order  to  eliminate  the  need  for 
recalculating  all  the  dimensions  of  a  casting,  pattern  makers  use  a  shrink  rule.  It  is 
longer  than  the  standard  1-foot  rule;  its  length  differs  for  the  different  metals  of  the 
casting. 

3.  Machine  finish  allowance.  The  dimensions  on  a  casting  are  oversized  to  compen- 
sate for  the  layer  of  metal  that  is  removed  through  subsequent  machining  to  obtain 
better  surface  finish. 

4.  Distortion  allowance.  Sometimes,  intricately  shaped  or  slender  castings  distort  dur- 
ing solidification,  even  though  reproduced  from  a  defect-free  pattern.  In  such  cases, 
it  is  necessary  to  distort  the  pattern  intentionally  to  obtain  a  casting  with  the  desired 
shape  and  dimensions. 

Cores  and  core  making.  Cores  are  the  parts  of  the  molds  that  form  desired  internal 
cavities,  recesses,  or  projections  in  castings.  A  core  is  usually  made  of  the  best  quality 
of  sand  to  have  the  shape  of  the  desired  cavity  and  is  placed  into  position  in  the  mold 
cavity.  Figure  3.1  shows  the  pattern,  mold,  and  core  used  for  producing  a  short  pipe 
with  two  flanges.  As  you  can  see,  projections,  called  core  prints,  are  added  to  both 
sides  of  the  pattern  to  create  impressions  that  allow  the  core  to  be  supported  and  held 
at  both  ends.  When  the  molten  metal  is  poured,  it  flows  around  the  core  to  fill  the  rest 
of  the  mold  cavity.  Cores  are  subjected  to  extremely  severe  conditions,  and  they  must, 
therefore,  possess  very  high  resistance  to  erosion,  exceptionally  high  strength,  good 
permeability,  good  refractoriness,  and  adequate  collapsibility  (i.e.,  the  rapid  loss  of 
strength  after  the  core  comes  in  contact  with  the  molten  metal).  Because  a  core  is  sur- 
rounded by  molten  metal  from  all  sides  (except  the  far  ends)  during  casting,  gases  have 
only  a  small  area  through  which  to  escape.  Therefore,  good  permeability  is  sometimes 


38 


3      Casting  and  Foundry  Work 


FIGURE    3.1 

The  pattern,  mold,  and 
core  used  for  producing 
a  short  pipe 


Core 


Pattern 

(before  removal 

from  mold) 


assisted  by  providing  special  vent  holes  to  allow  gases  to  escape  easily.  Another  re- 
quired characteristic  of  a  core  is  the  ability  to  shrink  in  volume  under  pressure  without 
cracking  or  failure.  The  importance  of  this  characteristic  is  obvious  when  you  consider 
a  casting  that  shrinks  onto  the  core  during  solidification.  If  the  core  is  made  hard 
enough  to  resist  the  shrinkage  of  the  casting,  the  latter  would  crack  as  a  result  of  being 
hindered  from  shrinking.  Figure  3.2  is  a  photograph  of  a  sand  core  for  an  automotive 
cam  tunnel. 

Core  sand  is  a  very  pure,  fine-grained  silica  sand  that  is  mixed  with  different 
binders,  depending  upon  the  casting  metal  with  which  it  is  going  to  be  used.  The 
binder  used  with  various  castings  includes  fireclay,  bentonite,  and  sodium  silicate  (in- 
organic binders),  as  well  as  oils  (cottonseed  or  linseed  oil),  molasses,  dexstrin,  and 
polymeric  resins  (organic  binders). 

Cores  are  usually  made  separately  in  core  boxes,  which  involve  cutting  or  ma- 
chining cavities  into  blocks  of  wood,  metal,  or  plastic.  The  surfaces  of  each  cavity 
must  be  very  smooth,  with  ample  taper  or  draft,  to  allow  easy  release  of  the  green  (un- 
baked) core.  Sometimes,  a  release  agent  is  applied  to  the  surfaces  of  the  cavity.  Core 
sand  is  rammed  into  the  cavity,  and  the  excess  is  then  struck  off  evenly  with  the  top  of 
the  core  box.  Next,  the  green  core  is  carefully  rolled  onto  a  metal  plate  and  is  baked  in 
an  oven.  Intricate  cores  are  made  of  separate  pieces  that  are  pasted  together  after  bak- 
ing. Sometimes,  cores  are  reinforced  with  annealed  low-carbon  steel  wires  or  even 


FIGURE    3.2 

Core  for  an  automotive 
cam  tunnel 


3.1  Classifications  of  Casting  by  Mold  Material 


39 


FIGURE    3.3 

A  simple  core  and  its 
corresponding  core  box 


1 

\ 

m 

Left  half  of 
core  box 


Right  half  of 
core  box 


Core 


Vent  hole 


cast-iron  grids  (in  the  case  of  large  cores)  to  ensure  coherence  and  stability.  Figure  3.3 
illustrates  a  simple  core  and  its  corresponding  core  box. 

Large,  round  cores  can  be  made  by  means  of  sweeps  or  templates,  and  drawing 
sweeps  are  employed  to  produce  large  cores  that  are  not  bodies  of  revolution.  Various 
machines  may  also  be  employed  in  the  core-making  process.  These  include  die  ex- 
truders, jolt-squeeze  machines,  sandslingers,  and  pneumatic  core  blowers.  Large  cores 
are  handled  in  the  foundry  and  placed  into  the  mold  by  means  of  a  crane. 

Gating  systems.  Molds  are  filled  with  molten  metal  by  means  of  channels,  called 
gates,  cut  in  the  sand  of  the  mold.  Figure  3.4  illustrates  a  typical  gating  system,  which 
includes  a  pouring  basin,  a  down  sprue,  a  sprue  base(well),  a  runner,  and  in-gates.  The 
design  of  the  gating  system  is  sometimes  critical  and  should,  therefore,  be  based  on  the 
theories  of  fluid  mechanics,  as  well  as  the  recommended  industrial  practice.  In  fact,  a 
gating  system  must  be  designed  so  that  the  following  are  ensured: 

1.  A  continuous,  uniform  flow  of  molten  metal  into  the  mold  cavity  must  be  pro- 
vided without  any  turbulence. 


FIGURE    3.4 

A  typical  gating  system 


Down  sprue 


n-gates 


Sprue  base  (well) 


Runner 


40  3      Casting  and  Foundry  Work 


2.  A  reservoir  of  molten  metal  that  feeds  the  casting  to  compensate  for  the  shrinkage 
during  solidification  must  be  maintained. 

3.  The  molten  metal  stream  must  be  prevented  from  separating  from  the  wall  of  the 
sprue. 

Let  us  now  break  down  the  gating  system  into  its  components  and  discuss  the  de- 
sign of  each  of  them.  The  pouring  basin  is  designed  to  reduce  turbulence.  The  molten 
metal  from  the  ladle  must  be  poured  into  the  basin  at  the  side  that  does  not  have  the 
tapered  sprue  hole.  The  hole  should  have  a  projection  with  a  generous  radius  around 
it,  as  shown  in  Figure  3.4,  in  order  to  eliminate  turbulence  as  the  molten  metal  enters 
the  sprue.  Next,  the  down  sprue  should  be  made  tapered  (its  cross-sectional  area 
should  decrease  when  going  downward)  to  prevent  the  stream  of  molten  metal  from 
separating  from  its  walls,  which  may  occur  because  the  stream  gains  velocity  as  it  trav- 
els downward  and,  therefore,  contracts  (remember  the  continuity  equation  in  fluid  me- 
chanics, Ax  V,  =  A2V2).  The  important  and  critical  element  of  the  gating  system  is  the 
in-gate,  whose  dimensions  affect  those  of  all  other  elements.  Sometimes,  the  cross- 
sectional  area  of  the  in-gate  is  reduced  in  the  zone  adjacent  to  the  sprue  base  to  create 
a  "choke  area"  that  is  used  mainly  to  control  the  flow  of  molten  metal  and,  conse- 
quently, the  pouring  time.  In  other  words,  it  serves  to  ensure  that  the  rate  of  molten- 
metal  flow  into  the  mold  cavity  is  not  higher  than  that  delivered  by  the  ladle  and, 
therefore,  keeps  the  gating  system  full  of  metal  throughout  the  casting  operation. 
On  the  other  hand,  gas  contamination,  slag  inclusions,  and  the  like  should  be  elim- 
inated by  maintaining  laminar  flow.  Accordingly,  the  Reynolds  number  (R„)  should 
be  checked  throughout  the  gating  system  (remember  that  the  flow  is  laminar  when 
/?„  <  2000).  Use  must  also  be  made  of  Bernoulli's  equation  to  calculate  the  velocity  of 
flow  at  any  cross  section  of  the  gating  system. 

In  some  cases,  when  casting  heavy  sections  or  high-shrinkage  alloys,  extra  reser- 
voirs of  molten  metal  are  needed  to  compensate  continually  for  the  shrinkage  of  the 
casting  during  solidification.  These  molten-metal  reservoirs  are  called  risers  and  are 
attached  to  the  casting  at  appropriate  locations  to  control  the  solidification  process.  The 
locations  of  the  feeding  system  and  the  risers  should  be  determined  based  on  the  phe- 
nomenon that  sections  most  distant  from  those  molten-metal  reservoirs  solidify  first. 
Risers  are  molded  into  the  cope  half  of  the  mold  to  ensure  gravity  feeding  of  the 
molten  metal  and  are  usually  open  to  the  top  surface  of  the  mold.  In  that  case,  they  are 
referred  to  as  open  risers.  When  they  are  not  open  to  the  top  of  the  mold,  they  are  then 
called  blind  risers.  Risers  can  also  be  classified  as  top  risers  and  side  risers,  depend- 
ing upon  their  location  with  respect  to  the  casting. 

Another  way  to  achieve  directional  solidification  is  the  use  of  chills;  these  involve 
inserts  of  steel,  cast  iron,  or  copper  that  act  as  a  "heat  sink"  to  increase  the  solidifica- 
tion rate  of  the  metal  at  appropriate  regions  of  the  casting.  Depending  upon  the  shape 
of  the  casting,  chills  can  be  external  or  internal. 

Molding  processes.  Green  sand  can  be  molded  by  employing  a  variety  of  processes, 
including  some  that  are  carried  out  both  by  hand  and  with  molding  machines.  Follow- 
ing is  a  brief  survey  of  the  different  green  sand  molding  methods: 


3.1  Classifications  of  Casting  by  Mold  Material 


41 


Flask  molding.  Flask  molding  is  the  most  widely  used  process  in  both  hand-  and 
machine-molding  practices.  Figure  3.5  illustrates  the  procedure  for  simple  hand- 
molding  using  a  single  (loose)  pattern.  First,  the  lower  half  of  the  pattern  is  placed 
on  a  molding  board  and  surrounded  by  the  drag.  The  drag  is  then  filled  with  sand 
(using  a  shovel)  and  rammed  very  firmly.  Ventilation  holes  are  made  using  a  steel 
wire,  but  these  should  not  reach  the  pattern.  The  drag  is  turned  upside  down  to 
bring  the  parting  plane  up  so  that  it  can  be  dusted.  Next,  the  other  half  of  the  pat- 
tern is  placed  in  position  to  match  the  lower  half,  and  the  cope  is  located  around  it, 
with  the  eyes  of  the  cope  fitted  to  the  pins  of  the  drag.  Sand  is  shoveled  into  the 
cope  and  rammed  firmly,  after  using  a  sprue  pin  to  provide  for  the  feeding  passage. 
Ventilation  holes  are  made  in  the  cope  part  of  the  mold  in  the  same  way  they  were 
made  in  the  other  half.  The  pouring  basin  is  cut  around  the  head  of  the  sprue  pin 
using  a  trowel,  and  the  sprue  pin  is  pulled  out  of  the  cope.  The  cope  is  then  care- 
fully lifted  off  the  drag  and  turned  so  that  the  parting  plane  is  upward.  The  two 
halves  of  the  pattern  are  removed  from  both  the  cope  and  the  drag.  The  runner 
and/or  gate  are  cut  from  the  mold  cavity  to  the  sprue  in  the  drag  part  of  the  mold. 
Then,  any  damages  are  repaired  by  slightly  wetting  the  location  and  using  a  slick. 
The  cope  is  then  carefully  placed  on  the  drag  to  assemble  the  two  halves  of  the 


FIGURE    3.5 

The  procedure  of  flask 
molding  using  a  single 
(loose)  pattern 


(1) 


(2) 


?zzzx; 


P 


(3) 


(4) 


42  3      Casting  and  Foundry  Work 


mold.  Finally,  the  cope  and  the  drag  are  fastened  together  by  means  of  shackles  or 
bolts  to  prevent  the  pressure  created  by  the  molten  metal  (after  pouring)  from  sep- 
arating them.  Enough  weight  can  be  placed  on  the  cope  as  an  alternative  to  using 
shackles  or  bolts.  In  fact,  the  pressure  of  the  molten  metal  after  casting  can  be  given 
by  the  following  equation: 

p  =  wx  h  (3.1) 

where:  p  is  the  pressure 

w  is  the  specific  weight  of  the  molten  metal 
h  is  the  height  of  the  cope 

The  force  that  is  trying  to  separate  the  two  halves  of  the  mold  can,  therefore,  be  given 
by  the  following  equation: 

F  =  p  x  A  (3.2) 

where:  F  is  the  force 

A  is  the  cross-sectional  area  of  the  casting  (including  the  runner,  gates, 
etc.)  at  the  parting  line 

2.  Stack  molding.  Stack  molding  is  best  suited  for  producing  a  large  number  of  small, 
light  castings  while  using  a  limited  amount  of  floor  space  in  the  foundry.  As  can  be 
seen  in  Figure  3.6a  and  b,  there  are  two  types  of  stack  molding:  upright  and 
stepped.  In  upright  stack  molding,  10  to  12  flask  sections  are  stacked  up.  They  all 
have  a  common  sprue  that  is  employed  in  feeding  all  cavities.  The  drag  cavity  is  al- 
ways molded  in  the  upper  surface  of  the  flask  section,  whereas  the  cope  cavity  is 
molded  in  the  lower  surface.  In  stepped  stack  molding,  each  section  has  its  own 
sprue  and  is,  therefore,  offset  from  the  one  under  it  to  provide  for  the  pouring  basin. 
In  this  case,  each  mold  is  cast  separately. 

3.  Sweep  molding.  Sweep  molding  is  used  to  form  the  surfaces  of  the  mold  cavity 
when  a  large-size  casting  must  be  produced  without  the  time  and  expenses  involved 
in  making  a  pattern.  A  sweep  that  can  be  rotated  around  an  axis  is  used  for  produc- 
ing a  surface  of  revolution,  contrary  to  a  drawing  sweep,  which  is  pushed  axially 
while  being  guided  by  a  frame  to  produce  a  surface  having  a  constant  section  along 
its  length  (see  discussion  of  the  extrusion  process  in  Chapter  5). 

4.  Pit  molding.  Pit  molding  is  usually  employed  for  producing  a  single  piece  of  a  large 
casting  when  it  would  be  difficult  to  handle  patterns  of  that  size  in  flasks.  Molding 
is  done  in  specially  prepared  pits  in  the  ground  of  the  foundry.  The  bottom  of  the 
pit  is  often  covered  with  a  layer  of  coke  that  is  2  to  3  inches  (50  to  75  mm)  thick. 
Then,  a  layer  of  sand  is  rammed  onto  the  coke  to  act  as  a  "bed"  for  the  mold.  Vent 
pipes  connect  the  coke  layer  to  the  ground  surface.  Molding  is  carried  out  as  usual, 
and  molds  are  almost  always  dried  before  pouring  the  molten  metal.  This  drying  is 
achieved  by  means  of  a  portable  mold  drier.  A  cope  that  is  also  dried  is  then  placed 
on  the  pit,  and  a  suitable  weight  or  a  group  of  weights  are  located  on  the  cope  to 
prevent  it  from  floating  when  the  molten  metal  is  poured. 


3.1  Classifications  of  Casting  by  Mold  Material 


43 


FIGURE    3.6 

The  two  types  of  stack 
molding:  (a)  upright; 
(b)  stepped 


(b) 


Molding  machines.  The  employment  of  molding  machines  results  in  an  increase  in 
the  production  rate,  a  marked  increase  in  productivity,  and  a  higher  and  more  con- 
sistent quality  of  molds.  The  function  of  these  machines  is  to  pack  the  sand  onto  the 
pattern  and  draw  the  pattern  out  from  the  mold.  There  are  several  types  of  molding 
machines,  each  with  a  different  way  of  packing  the  sand  to  form  the  mold.  The  main 
types  include  squeezers,  jolt  machines,  and  sandslingers.  There  are  also  some  ma- 
chines, such  as  jolt-squeeze  machines,  that  employ  a  combination  of  the  working 


44 


3      Casting  and  Foundry  Work 


principles  of  two  of  the  main  types.  Following  is  a  brief  discussion  of  the  three  main 
types  of  molding  machines  (see  Figure  3.7): 

1.  Squeezers.  Figure  3.7a  illustrates  the  working  principle  of  the  squeezer  type  of 
molding  machine.  The  pattern  plate  is  clamped  on  the  machine  table,  and  a  flask  is 
put  into  position.  A  sand  frame  is  placed  on  the  flask,  and  both  are  then  filled  with 


FIGURE    3.7 

Molding  machines: 

(a)  squeezer; 

(b)  jolt  machine; 

(c)  sandslinger 


Squeeze 
head 


Flask 


(0 


3.1  Classifications  of  Casting  by  Mold  Material  45 

sand  from  a  hopper.  Next,  the  machine  table  travels  upward  to  squeeze  the  sand  be- 
tween the  pattern  plate  and  a  stationary  head.  The  squeeze  head  enters  into  the  sand 
frame  and  compacts  the  sand  so  that  it  is  level  with  the  edge  of  the  flask. 

2.  Jolt  machines.  Figure  3.7b  illustrates  the  working  principle  of  the  jolt  type  of 
molding  machine.  As  can  be  seen,  compressed  air  is  admitted  through  the  hose  to  a 
pressure  cylinder  to  lift  the  plunger  (and  the  flask,  which  is  full  of  sand)  up  to  a  cer- 
tain height,  where  the  side  hole  is  uncovered  to  exhaust  the  compressed  air.  The 
plunger  then  falls  down  and  strikes  the  stationary  guiding  cylinder.  The  shock  wave 
resulting  from  each  of  the  successive  impacts  contributes  to  packing  the  molding 
sand  in  the  flask. 

3.  Sandslingers.  Figure  3.7c  shows  a  sandslinger.  This  type  of  machine  is  employed 
in  molding  sand  in  flasks  of  any  size,  whether  for  individual  or  mass  production  of 
molds.  Sandslingers  are  characterized  by  their  high  output,  which  amounts  to  2500 
cubic  feet  (more  than  60  cubic  meters)  per  hour.  As  can  be  seen,  molding  sand  is 
fed  into  a  housing  containing  an  impeller  that  rotates  rapidly  around  a  horizontal 
axis.  Sand  particles  are  picked  up  by  the  rotating  blades  and  thrown  at  a  high  speed 
through  an  opening  onto  the  pattern,  which  is  located  in  the  flask. 

No  matter  what  type  of  molding  machine  is  used,  special  machines  are  employed 
to  draw  the  pattern  out  of  the  mold.  Basically,  these  machines  achieve  that  goal  by 
turning  the  flask  (together  with  the  pattern)  upside  down  and  then  lifting  the  pattern 
out  of  the  mold.  Examples  of  these  machines  include  roll-over  molding  machines  and 
rock-over  pattern-draw  machines. 

Sand  conditioning.  The  molding  sand,  whether  new  or  used,  must  be  conditioned  be- 
fore being  used.  When  used  sand  is  to  be  recycled,  lumps  should  be  crushed  and  then 
metal  granules  or  small  parts  removed  (a  magnetic  field  is  employed  in  a  ferrous 
foundry).  Next,  sand  (new  or  recycled)  and  all  other  molding  constituents  must  be 
screened  in  shakers,  rotary  screens,  or  vibrating  screens.  Molding  materials  are  then 
thoroughly  mixed  in  order  to  obtain  a  completely  homogeneous  green  sand  mixture. 
The  more  uniform  the  distribution,  the  better  the  molding  properties  (like  permeability 
and  green  strength)  of  the  sand  mixture  will  be. 

Mixing  is  carried  out  in  either  continuous-screw  mixers  or  vertical-wheel  mullers. 
The  mixers  mix  the  molding  materials  by  means  of  two  large  screws  or  worm  gears; 
the  mullers  are  usually  used  for  batch-type  mixing.  A  typical  muller  is  illustrated  in 
Figure  3.8.  It  consists  primarily  of  a  pan  in  which  two  wheels  rotate  about  their  own 
horizontal  axis  as  well  as  about  a  stationary  vertical  shaft.  Centrifugal  mullers  are  also 
in  use,  especially  for  high  production  rates. 

Dry  Sand  Molds 

As  previously  mentioned,  green  sand  molds  contain  up  to  8  percent  water,  depending 
upon  the  kind  and  percentage  of  the  binding  material.  Therefore,  this  type  of  mold  can 
be  used  only  for  small  castings  with  thin  walls;  large  castings  with  thick  walls  would 
heat  the  mold,  resulting  in  vaporization  of  water,  which  would,  in  turn,  lead  to  bubbles 


46 


3      Casting  and  Foundry  Work 


FIGURE    3.8 

A  muller  for  sand 
conditioning 


Plow  blade 


Outlet 

(conditioned  sand) 


Wheels 


in  the  castings.  For  this  reason,  molds  for  large  castings  should  be  dried  after  they 
are  made  in  the  same  way  as  green  sand  molds.  The  drying  operation  is  carried  out 
in  ovens  at  temperatures  ranging  from  300°F  to  650°F  (150°C  to  350°C)  for  8  up  to 
48  hours,  depending  upon  the  kind  and  amount  of  binder  used. 

Core-Sand  Molds 

When  the  mold  is  too  big  to  fit  in  an  oven,  molds  are  made  by  assembling  several 
pieces  of  sand  cores.  Consequently,  patterns  are  not  required,  and  core  boxes  are  em- 
ployed instead  to  make  the  different  sand  cores  necessary  for  constructing  the  mold. 
Because  core-sand  mixtures  (which  have  superior  molding  properties)  are  used,  very 
good  quality  and  dimensional  accuracy  of  the  castings  are  obtained. 

Cement-Bonded  Sand  Molds 

A  mixture  of  silica  sand  containing  8  to  12  percent  cement  and  4  to  6  percent  water  is 
used.  When  making  the  mold,  the  cement-bonded  sand  mixture  must  be  allowed  to 
harden  first  before  the  pattern  is  withdrawn.  The  obtained  mold  is  then  allowed  to  cure 
for  about  3  to  5  days.  Large  castings  with  intricate  shapes,  accurate  dimensions,  and 
smooth  surfaces  are  usually  produced  in  this  way,  the  only  shortcoming  being  the  long 
time  required  for  the  molding  process. 

Carbon  Dioxide  Process  for  Molding 

Silica  sand  is  mixed  with  a  binder  involving  a  solution  of  sodium  silicate  (water  glass) 
amounting  to  6  percent.  After  the  mold  is  rammed,  carbon  dioxide  is  blown  through 
the  sand  mixture.  As  a  result,  the  gel  of  silica  binds  the  sand  grains  together,  and  no 


3.1  Classifications  of  Casting  by  Mold  Material  47 

drying  is  needed.  Because  the  molds  are  allowed  to  harden  while  the  pattern  is  in  po- 
sition, high  dimensional  accuracy  of  molds  is  obtained. 

Plaster  Molds 

A  plaster  mold  is  appropriate  for  casting  silver,  gold,  magnesium,  copper,  and  alu- 
minum alloys.  The  molding  material  is  a  mixture  of  fine  silica  sand,  asbestos,  and  plas- 
ter of  paris  as  a  binder.  Water  is  added  to  the  mixture  until  a  creamy  slurry  is  obtained, 
which  is  then  employed  in  molding.  The  drying  process  should  be  very  slow  to  avoid 
cracking  of  the  mold. 

Loam  Molds 

The  loam  mold  is  used  for  very  large  jobs.  The  basic  shape  of  the  desired  mold  is  con- 
structed with  bricks  and  mortar  (just  like  a  brick  house).  A  loam  mixture  is  then  used 
as  a  molding  material  to  obtain  the  desired  fine  details  of  mold.  Templates,  sweeps, 
and  the  like  are  employed  in  the  molding  process.  The  loam  mixture  used  in  molding 
consists  of  50  percent  or  more  of  loam,  with  the  rest  being  mainly  silica  sand.  Loam 
molds  must  be  thoroughly  dried  before  pouring  the  molten  metal. 

Shell  Molds 

In  shell  molding,  a  thin  mold  is  made  around  a  heated-metal  pattern  plate.  The  mold- 
ing material  is  a  mixture  of  dry,  fine  silica  sand  (with  a  very  low  clay  content)  and  3 
to  8  percent  of  a  thermosetting  resin  like  phenolformaldehyde  or  ureaformaldehyde. 
Conventional  dry-mixing  techniques  are  used  for  obtaining  the  molding  mixture.  Spe- 
cially prepared  resin-coated  sands  are  also  used. 

When  the  molding  mixture  drops  onto  the  pattern  plate,  which  is  heated  to  a  tem- 
perature of  350°F  to  700°F  (180°C  to  375°C),  a  shell  about  1/4  inch  (6  mm)  thick  is 
formed.  In  order  to  cure  the  shell  completely,  it  must  be  heated  at  450°F  to  650°F 
(230°C  to  350°C)  for  about  1  to  3  minutes.  The  shell  is  then  released  from  the  pattern 
plate  by  ejector  pins.  To  prevent  sticking  of  the  baked  shell,  sometimes  called  the  bis- 
cuit, to  the  pattern  plate,  a  silicone  release  agent  is  applied  to  the  plate  before  the  mold- 
ing mixture  drops  onto  it.  Figure  3.9  is  a  photograph  of  a  pattern  of  a  crankshaft  used 
in  shell  molding. 

Shell  molding  is  suitable  for  mass  production  of  thin-walled,  gray  cast-iron  (and 
aluminum-alloy)  castings  having  a  maximum  weight  between  35  and  45  pounds  (15 
and  20  kg).  However,  castings  weighing  up  to  1000  pounds  (450  kg)  can  be  made  by 
employing  shell  molding  on  an  individual  basis.  The  advantages  of  shell  molding  in- 
clude good  surface  finish,  few  restrictions  on  casting  design,  and  the  fact  that  this 
process  renders  itself  suitable  for  automation. 

Ceramic  Molds 

In  the  ceramic  molding  process,  the  molding  material  is  actually  a  slurry  consisting 
of  refractory  grains,  ceramic  binder,  water,  alcohol,  and  an  agent  to  adjust  the  pH 
value  (see  discussion  of  slurry  casting  in  Chapter  7).  The  slurry  is  poured  around  the 


48 


3      Casting  and  Foundry  Work 


FIGURE    3.9 

A  pattern  of  a 
crankshaft  used  in  shell 
molding 


m\    If     rlt-LJ  I    II        W     I  ^H 


permanent  (reusable)  pattern  and  is  allowed  to  harden  when  the  pattern  is  withdrawn. 
Next,  the  mold  is  left  to  dry  for  some  time  and  then  is  fired  to  gain  strength.  In  fact, 
ceramic  molds  are  usually  preheated  before  pouring  the  molten  metal.  For  this  rea- 
son, they  are  suitable  for  casting  high-pouring-temperature  alloys.  Excellent  surface 
finish  and  very  close  tolerances  of  the  castings  are  among  the  advantages  of  this 
molding  process  and  lead  to  the  elimination  of  the  machining  operations  that  are  usu- 
ally performed  on  castings.  Therefore,  ceramic  molds  are  certainly  advantageous 
when  casting  precious  or  difficult-to-machine  metals  as  well  as  for  making  castings 
with  great  shape  intricacy. 

Precision  Molds  (Investment  Casting) 

Precision  molding  is  used  when  castings  with  intricate  shapes,  good  dimensional  ac- 
curacy, and  very  smooth  surfaces  are  required.  The  process  is  especially  advantageous 
for  high-melting-point  alloys  as  well  as  for  difficult-to-machine  metals.  It  is  also  most 
suitable  for  producing  small  castings  having  intricate  shapes,  such  as  the  group  of  in- 
vestment castings  shown  in  Figure  3.10.  A  nonpermanent  pattern  that  is  usually  made 
of  wax  must  be  prepared  for  each  casting.  Therefore,  the  process  is  sometimes  referred 
to  as  the  lost-wax  process.  Generally,  the  precision  molding  process  involves  the  fol- 
lowing steps  (see  Figure  3.11): 

1.  A  heat-disposable  pattern,  together  with  its  gating  system,  is  prepared  by  injecting 
wax  or  plastic  into  a  die  cavity. 

2.  A  pattern  assembly  that  is  composed  of  a  number  of  identical  patterns  is  made.  Pat- 
terns are  attached  to  a  runner  bar  made  of  wax  or  plastic  in  much  the  same  manner 
as  leaves  are  attached  to  branches.  A  ceramic  pouring  cup  is  also  attached  to  the  top 
of  the  pattern  assembly,  which  is  sometimes  referred  to  as  the  tree  or  cluster  (see 
Figure  3.11a). 


3.1  Classifications  of  Casting  by  Mold  Material 


49 


FIGURE    3.10 

A  group  of  investment 
castings  (Courtesy  of 
Fansteel  ESCAST, 
Addison,  Illinois) 


4h 


'^*M>\ 


\        \ 


3.  The  tree  is  then  invested  by  separately  dipping  it  into  a  ceramic  slurry  that  is  com- 
posed of  silica  flour  suspended  in  a  solution  of  ethyl  silicate  and  sprinkling  it  with 
very  fine  silica  sand.  A  self-supporting  ceramic  shell  mold  about  1/4  inch  (6  mm) 
thick  is  formed  all  around  the  wax  assembly  (see  Figure  3.1  lb).  Alternatively,  a  thin 
ceramic  precoating  is  obtained,  and  then  the  cluster  is  placed  in  a  flask  and  a  thick 
slurry  is  poured  around  it  as  a  backup  material. 

4.  The  pattern  assembly  is  then  baked  in  an  oven  or  a  steam  autoclave  to  melt  out  the 
wax  (or  plastic).  Therefore,  the  dimensions  of  the  mold  cavity  precisely  match 
those  of  the  desired  product. 

5.  The  resulting  shell  mold  is  fired  at  a  temperature  ranging  from  1600°F  to  1800°F 
(900°C  to  1000°C)  to  eliminate  all  traces  of  wax  and  to  gain  reasonable  strength. 

6.  The  molten  metal  is  poured  into  the  mold  while  the  mold  is  still  hot,  and  a  cluster 
of  castings  is  obtained  (see  Figure  3.11c). 

Today,  the  lost-wax  process  is  used  in  manufacturing  large  objects  like  cylinder 
heads  and  camshafts.  The  modern  process,  which  is  known  as  the  lost-foam  method, 
involves  employing  a  styrofoam  replica  of  the  finished  product,  which  is  then  coated 
with  a  refractory  material  and  located  in  a  box,  where  sand  is  molded  around  it  by  vi- 
bratory compaction.  When  the  molten  metal  is  finally  poured  into  the  mold,  the  styro- 
foam vaporizes,  allowing  the  molten  metal  to  replace  it. 


50 


3      Casting  and  Foundry  Work 


FIGURE    3.11 

Steps  involved  in  investment  casting:  (a)  a  cluster  of  wax  patterns;  (b)  a  cluster  of  ceramic  shells; 
(c)  a  cluster  of  castings  (Courtesy  of  Fansteel  ESCAST,  Addison,  Illinois) 


Graphite  Molds 

Graphite  is  used  in  making  molds  to  receive  alloys  (such  as  titanium)  that  can  be 
poured  only  into  inert  molds.  The  casting  process  must  be  performed  in  a  vacuum  to 
eliminate  any  possibility  of  contaminating  the  metal.  Graphite  molds  can  be  made  ei- 
ther by  machining  a  block  of  graphite  to  create  the  desired  mold  cavity  or  by  com- 
pacting a  graphite-base  aggregate  around  the  pattern  and  then  sintering  the  obtained 
mold  at  a  temperature  of  18()0°F  to  2000°F  (1000°C  to  1120°C)  in  a  reducing  atmos- 
phere (see  Chapter  7).  In  fact,  graphite  mold  liners  have  found  widespread  industrial 
application  in  the  centrifugal  casting  of  brass  and  bronze. 


3.1  Classifications  of  Casting  by  Mold  Material 


51 


Permanent  Molds 

A  permanent  mold  can  be  used  repeatedly  for  producing  castings  of  the  same  form  and 
dimensions.  Permanent  molds  are  usually  made  of  steel  or  gray  cast  iron.  Figure  3.12a 
and  b  shows  a  permanent  mold  made  of  alloy  steel  for  molding  a  cylinder  block.  Each 
mold  is  generally  made  of  two  or  more  pieces  that  are  assembled  together  by  fitting 
and  clamping.  Although  the  different  parts  of  the  mold  can  be  cast  to  their  rough  con- 
tours, subsequent  machining  and  finishing  operations  are  necessary  to  eliminate  the 
possibility  of  the  casting's  sticking  to  the  mold.  Simple  cores  made  of  metal  are  fre- 
quently used.  When  complex  cores  are  required,  they  are  usually  made  of  sand  or  plas- 
ter, and  the  mold  is  said  to  be  semipermanent. 

Different  metals  and  alloys  can  successfully  be  cast  in  permanent  molds.  They  in- 
clude aluminum  alloys,  magnesium  alloys,  zinc  alloys,  lead,  copper  alloys,  and  cast 


FIGURE    3.12 

A  permanent  mold 
made  of  alloy  steel  for 
casting  a  cylinder  block: 
(a) drag;  (b)  cope 


52  3      Casting  and  Foundry  Work 


irons.  It  is  obvious  that  the  mold  should  be  preheated  to  an  appropriate  temperature 
prior  to  casting.  In  fact,  the  operating  temperature  of  the  mold,  which  depends  upon 
the  metal  to  be  cast,  is  a  very  important  factor  in  successful  permanent-mold  casting. 

Based  on  the  preceding  discussion,  we  can  expect  the  mold  life  to  be  dependent 
upon  a  number  of  interrelated  factors,  including  the  mold  material,  the  metal  to  be  cast, 
and  the  operating  temperature  of  the  mold.  Nevertheless,  it  can  be  stated  that  the  life 
of  a  permanent  mold  is  about  100,000  pourings  or  more  when  casting  zinc,  magne- 
sium, or  aluminum  alloys  and  not  more  than  20,000  pourings  for  copper  alloys  and 
cast  irons.  However,  mold  life  can  be  extended  by  spraying  the  surface  of  the  mold 
cavity  with  colloidal  refractories  suspended  in  liquids. 

The  advantages  of  permanent-mold  casting  include  substantial  increases  in  pro- 
ductivity (a  mold  does  not  have  to  be  made  for  each  casting),  close  tolerances,  supe- 
rior surface  finish,  and  improved  mechanical  properties  of  the  castings.  A  further 
advantage  is  the  noticeable  reduction  in  the  percentage  of  rejects  when  compared  with 
the  conventional  sand-casting  processes.  Nevertheless,  the  process  is  economically 
feasible  for  mass  production  only.  There  is  also  a  limitation  on  the  size  of  parts  pro- 
duced by  permanent-mold  casting.  A  further  limitation  is  that  not  all  alloys  are  suited 
to  this  process. 


3.2  CLASSIFICATIONS  OF  CASTING 

BY  METHOD  OF  FILLING  THE  MOLD 


For  all  types  of  molds  that  we  have  discussed,  the  molten  metal  is  almost  always  fed 
into  the  mold  only  by  the  action  of  gravity.  Therefore,  the  casting  process  is  referred 
to  as  gravity  casting.  There  are,  however,  other  special  ways  of  pouring  or  feeding  the 
molten  metal  into  the  desired  cavities.  These  casting  methods  are  generally  aimed  at 
forcing  the  molten  metal  to  flow  and  fill  the  fine  details  of  the  mold  cavity  while  elim- 
inating the  internal  defects  experienced  in  conventional  gravity  casting  processes.  Fol- 
lowing is  a  survey  of  the  commonly  used  special  casting  processes. 

Die  Casting 

Die  casting  involves  forcing  the  molten  metal  into  the  cavity  of  a  steel  mold,  called  a 
die,  under  very  high  pressure  (1000  to  30,000  pounds  per  square  inch,  or  about  70  to 
2000  times  the  atmospheric  pressure).  In  fact,  this  characteristic  is  the  major  difference 
between  die  casting  and  permanent-mold  casting,  where  the  molten  metal  is  fed  into 
the  mold  either  by  gravity  or  at  low  pressures.  Die  casting  may  be  classified  according 
to  the  type  of  machine  used.  The  two  principal  types  are  hot-chamber  machines  and 
cold-chamber  machines. 

Hot-chamber  machines.  The  main  components  of  the  hot-chamber  die  casting  ma- 
chine include  a  steel  pot  filled  with  the  molten  metal  to  be  cast  and  a  pumping  system 
that  consists  of  a  pressure  cylinder,  a  plunger,  a  gooseneck  passage,  and  a  nozzle.  With 
the  plunger  in  the  up  position,  as  shown  in  Figure  3.13a,  the  molten  metal  flows  by 
gravity  through  the  intake  ports  into  the  submerged  hot  chamber.  When  the  plunger  is 


3.2  Classifications  of  Casting  by  Method  of  Filling  the  Mold 


53 


FIGURE    3.13 

The  hot-chamber  die  casting  method:  (a)  filling  the  chamber;  (b)  metal  forced  into  the  die  cavity 


Plunger 


Hot  pot 


Inlets 


Burner 


Movable 

die  ^f  Stationary 

die  half 


(a) 


(b) 


pushed  downward  by  the  power  cylinder  (not  shown  in  the  figure),  it  shuts  off  the  in- 
take port.  Then,  with  further  downward  movement,  the  molten  metal  is  forced  through 
the  gooseneck  passage  and  the  nozzle  into  the  die  cavity,  as  shown  in  Figure  3.13b. 
Pressures  ranging  from  700  to  2000  pounds  per  square  inch  (50  to  150  atmospheres) 
are  quite  common  to  guarantee  complete  filling  of  the  die  cavity.  After  the  cavity  is  full 
of  molten  metal,  the  pressure  is  maintained  for  a  preset  dwell  time  to  allow  the  casting 
to  solidify  completely.  Next,  the  two  halves  of  the  die  are  pushed  apart,  and  the  cast- 
ing is  knocked  out  by  means  of  ejector  pins.  The  die  cavity  is  then  cleaned  and  lubri- 
cated before  the  cycle  is  repeated. 

The  advantages  of  hot-chamber  die  casting  are  numerous.  They  include  high  pro- 
duction rates  (especially  when  multicavity  dies  are  used),  improved  productivity,  su- 
perior surface  finish,  very  close  tolerances,  and  the  ability  to  produce  intricate  shapes 
with  thin  walls.  Nevertheless,  the  process  has  some  limitations.  For  instance,  only  low- 
melting-point  alloys  (such  as  zinc,  tin,  lead,  and  the  like)  can  be  cast  because  the  com- 
ponents of  the  pumping  system  are  in  direct  contact  with  the  molten  metal  throughout 
the  process.  Also,  die  casting  is  usually  only  suitable  for  producing  small  castings  that 
weigh  less  than  10  pounds  (4.5  kg). 

Cold-chamber  machines.  In  the  cold-chamber  die  casting  machine,  the  molten-metal 
reservoir  is  separate  from  the  casting  machine,  and  just  enough  for  one  shot  of 
molten  metal  is  ladled  every  stroke.  Consequently,  the  relatively  short  exposure  of 
the  shot  chamber  and  the  plunger  to  the  molten  metal  allows  die  casting  of  alu- 
minum, magnesium,  brass,  and  other  alloys  having  relatively  high  melting  points.  In 
the  sequence  of  operations  in  cold-chamber  die  casting,  the  molten  metal  is  first  la- 
dled through  the  pouring  hole  of  the  shot  chamber  while  the  two  halves  of  the  die  are 
closed  and  locked  together,  as  shown  in  Figure  3.14.  Next,  the  plunger  moves  for- 
ward to  close  off  the  pouring  hole  and  then  forces  the  molten  metal  into  the  die  cav- 
ity. Pressures  in  the  shot  chamber  may  go  over  30,000  pounds  per  square  inch  (2000 


54 


3      Casting  and  Foundry  Work 


FIGURE    3.14 

The  cold-chamber  die 
casting  method 


Ejector 


Ladle 


Plunger 


atmospheres).  After  the  casting  has  solidified,  the  two  halves  of  the  die  are  opened, 
and  the  casting,  together  with  the  gate  and  the  slug  of  excess  metal,  are  ejected  from 
the  die. 

It  is  not  difficult  to  see  that  large  parts  weighing  50  pounds  (23  kg)  can  be  pro- 
duced by  cold-chamber  die  casting.  The  process  is  very  successful  when  casting  alu- 
minum alloys,  copper  alloys,  and  high-temperature  aluminum-zinc  alloys.  However, 
this  process  has  a  longer  cycle  time  when  compared  with  hot-chamber  die  casting.  A 
further  disadvantage  is  the  need  for  an  auxiliary  system  for  pouring  the  molten  metal. 
It  is  mainly  for  this  reason  that  vertical  cold-chamber  machines  were  developed.  As 
can  be  seen  in  Figure  3.15,  such  a  machine  has  a  transfer  tube  that  is  submerged  into 
molten  metal.  It  is  fed  into  the  shot  chamber  by  connecting  the  die  cavity  to  a  vacuum 
tank  by  means  of  a  special  valve.  The  molten  metal  is  forced  into  the  die  cavity  when 
the  plunger  moves  upward. 

Centrifugal  Casting 

Centrifugal  casting  refers  to  a  group  of  processes  in  which  the  forces  used  to  distrib- 
ute the  molten  metal  in  the  mold  cavity  (or  cavities)  are  caused  by  centrifugal  acceler- 
ation. Centrifugal  casting  processes  can  be  classified  as  true  centrifugal  casting, 


FIGURE    3.15 

A  vertical  cold-chamber 
die  casting  machine 


Vacuum 


Vacuum 


3.2  Classifications  of  Casting  by  Method  of  Filling  the  Mold 


55 


semicentrifugal  casting,  and  the  centrifuging  method.  Each  of  these  processes  is  briefly 
discussed  next. 

True  centrifugal  casting.  True  centrifugal  casting  involves  rotating  a  cylindrical  mold 
around  its  own  axis,  with  the  revolutions  per  minute  high  enough  to  create  an  effective 
centrifugal  force,  and  then  pouring  molten  metal  into  the  mold  cavity.  The  molten  metal 
is  pushed  to  the  walls  of  the  mold  by  centrifugal  acceleration  (usually  70  to  80  times  that 
of  gravity),  where  it  solidifies  in  the  form  of  a  hollow  cylinder.  The  outer  shape  of  the 
casting  is  given  by  the  mold  contour,  while  the  diameter  of  the  inner  cylindrical  surface 
is  controlled  by  the  amount  of  molten  metal  poured  into  the  mold  cavity.  The  machines 
used  to  spin  the  mold  may  have  either  horizontal  or  vertical  axes  of  rotation.  Short  tubes 
are  usually  cast  in  vertical-axis  machines,  whereas  longer  pipes,  like  water  supply  and 
sewer  pipes,  are  cast  using  horizontal-axis  machines.  The  basic  features  of  a  true  cen- 
trifugal casting  machine  with  a  horizontal  axis  are  shown  in  Figure  3.16. 

Centrifugal  castings  are  characterized  by  their  high  density,  refined  fine-grained 
structure,  and  superior  mechanical  properties,  accompanied  by  a  low  percentage  of  re- 
jects and,  therefore,  a  high  production  output.  A  further  advantage  of  the  centrifugal 
casting  process  is  the  high  efficiency  of  metal  utilization  due  to  the  elimination  of 
sprues  and  risers  and  the  small  machining  allowance  used. 

Semicentrifugal  casting.  Semicentrifugal  casting  is  quite  similar  to  the  preceding 
type,  the  difference  being  that  the  mold  cavity  is  completely  filled  with  the  molten 
metal.  But  because  centrifugal  acceleration  is  dependent  upon  the  radius,  the  central 
core  of  the  casting  is  subjected  to  low  pressure  and  is,  therefore,  the  region  where  en- 
trapped air  and  inclusions  are  present.  For  this  reason,  the  semicentrifugal  casting 
process  is  recommended  for  producing  castings  that  are  to  be  subjected  to  subsequent 
machining  to  remove  their  central  cores.  Examples  include  cast  track  wheels  for  tanks, 


FIGURE    3.16 

A  true  centrifugal 
casting  machine 


Motor 


Spacers 


Ladle 


56  3      Casting  and  Foundry  Work 


tractors,  and  the  like.  A  sand  core  is  sometimes  used  to  form  the  central  cavity  of  the 
casting  in  order  to  eliminate  the  need  for  subsequent  machining  operations. 

Centrifuging.  In  the  centrifuging  method,  a  number  of  mold  cavities  are  arranged  on 
the  circumference  of  a  circle  and  are  connected  to  a  central  down  sprue  through  radial 
gates.  Next,  molten  metal  is  poured,  and  the  mold  is  rotated  around  the  central  axis  of 
the  sprue.  In  other  words,  each  casting  is  rotated  around  an  axis  off  (shifted  from)  its 
own  center  axis.  Therefore,  mold  cavities  are  filled  under  high  pressure,  so  the  process 
is  usually  used  for  producing  castings  with  intricate  shapes;  the  increased  pressure  on 
the  casting  during  solidification  allows  the  fine  details  of  the  mold  to  be  obtained. 


Continuous  Casting 

The  continuous  casting  process  is  gaining  widespread  industrial  use,  especially  for 
high-quality  alloy  steel.  In  fact,  the  process  itself  passed  through  a  few  evolutionary 
stages.  Although  it  was  originally  developed  for  producing  cast-iron  sheets,  an  up-to- 
date  version  is  now  being  used  for  casting  semifinished  products  that  are  to  be 
processed  subsequently  by  piercing,  forging,  extrusion,  and  the  like. 

The  continuous  casting  process  basically  involves  controlling  the  flow  of  a  stream 
of  molten  metal  that  comes  out  from  a  water-cooled  orifice  in  order  to  solidify  and 
form  a  continuous  strip  (or  rod).  The  new  version  of  this  process  is  usually  referred  to 
as  rotary  continuous  casting  because  the  water-cooled  mold  (orifice)  is  always  oscil- 
lating and  rotating  at  about  120  revolutions  per  minute  during  casting.  Figure  3.17  il- 
lustrates the  principles  of  rotary  continuous  casting.  The  steel  is  melted,  refined,  and 
degassed  and  its  chemical  composition  controlled  before  it  is  transferred  and  poured 
into  the  caster  (tundish).  The  molten  metal  then  enters  the  rotating  mold  tangent  to  the 
edge  through  the  bent  tube.  The  centrifugal  force  then  forces  the  steel  against  the  mold 
wall,  while  lighter  inclusions  and  impurities  remain  in  the  center  of  the  vortex,  where 
they  are  removed  by  the  operator.  Solidification  of  the  metal  flowing  out  of  the  mold 
continues  at  a  precalculated  rate.  The  resulting  bar  is  then  cut  by  a  circular  saw  that  is 
traveling  downward  at  the  same  speed  as  the  bar.  The  bar  is  tilted  and  loaded  onto  a 
conveyor  to  transfer  it  to  the  cooling  bed  and  the  rolling  mill. 

The  continuous  casting  process  has  the  advantages  of  very  high  metal  yield 
(about  98  percent,  compared  with  87  percent  in  conventional  ingot-mold  practice), 
excellent  quality  of  cast,  controlled  grain  size,  and  the  possibility  of  casting  special 
cross-sectional  shapes. 


The  V-Process 

The  vacuum  casting  process  (V-process  for  short)  involves  covering  the  two  halves  of 
the  pattern  with  two  plastic  films  that  are  0.005  inch  (0.125  mm)  thick  by  employing 
vacuum  forming  (see  chapter  8).  The  pattern  is  then  removed,  and  the  two  formed- 
plastic  sheets  are  tightened  together  to  form  a  mold  cavity  that  is  surrounded  by  a  flask 
filled  with  sand  (there  is  no  need  for  a  binder).  This  mold  cavity  is  kept  in  a  vacuum 
as  the  molten  metal  is  poured  to  assist  and  ensure  easy  flow. 


3.2  Classifications  of  Casting  by  Method  of  Filling  the  Mold 


57 


FIGURE    3.17 

The  principles  of  rotary 
continuous  casting 


Bent  tube 


0 
0 


O 
O 


0 


0 


O 
0 


Ladle 


Ceramic  tube 


Tundish 


Casting 


Guiding  rolls 


Extractor 


Hot  saw  (it  travels 

downwards 

while  cutting) 


Product 


Conveyor  to  rolling  mill 


The  V-process,  developed  in  Japan  in  the  early  1970s,  offers  many  advantages, 
such  as  the  elimination  of  the  need  for  special  molding  sands  with  binders  and  the 
elimination  of  the  problems  associated  with  green  sand  molding  (like  gas  bubbles 
caused  by  excess  humidity).  Also,  the  size  of  risers,  vents,  and  sprues  can  be  reduced 
markedly,  thus  resulting  in  an  increase  in  the  efficiency  of  material  utilization.  Figure 
3.18  shows  a  plastic  mold  being  prepared  for  the  V-process. 


58 


3      Casting  and  Foundry  Work 


FIGURE    3.18 

A  plastic  mold  being  prepared  for  the  V-process  (Courtesy  of  Spectrum  Casting,  Inc.,  Flint, 
Michigan) 


3.3  CLASSIFICATIONS  OF  CASTING 
BY  METAL  TO  BE  CAST 


When  classified  by  metal,  castings  can  be  either  ferrous  or  nonferrous.  The  ferrous 
castings  include  cast  steels  and  the  family  of  cast  irons,  whereas  the  nonferrous  cast- 
ings include  all  other  metals,  such  as  aluminum,  copper,  magnesium,  titanium,  and 
their  alloys.  Each  of  these  metals  and  alloys  is  melted  in  a  particular  type  of  foundry 
furnace  that  may  not  be  appropriate  for  melting  other  metals  and  alloys.  Also,  molding 
methods  and  materials,  as  well  as  fluxes,  degassers,  and  additives,  depend  upon  the 
metal  to  be  cast.  Therefore,  this  classification  method  is  popular  in  foundry  work.  Fol- 
lowing is  a  brief  discussion  of  each  of  these  cast  alloys. 

Ferrous  Metals 

Cast  steels.  Steels  are  smelted  in  open-hearth  furnaces,  convertors,  electric-arc  fur- 
naces, and  electric-induction  furnaces.  Cast  steels  can  be  either  plain-carbon,  low- 
alloy,  or  high-alloy  steel.  However,  plain-carbon  cast  steel  is  the  most  commonly 
produced  type.  When  compared  with  cast  iron,  steel  certainly  has  poorer  casting  prop- 
erties— namely,  higher  melting  point,  higher  shrinkage,  and  poorer  fluidity.  Steels  are 
also  more  susceptible  to  hot  and  cold  cracks  after  the  casting  process.  Therefore,  cast 


3.3  Classifications  of  Casting  by  Metal  to  Be  Cast  59 

steels  are  almost  always  subjected  to  heat  treatment  to  relieve  the  internal  stresses  and 
improve  the  mechanical  properties. 

In  order  to  control  the  oxygen  content  of  molten  steels,  aluminum,  silicon,  or  man- 
ganese is  used  as  a  deoxidizer.  Aluminum  is  the  most  commonly  used  of  these  ele- 
ments because  of  its  availability,  low  cost,  and  effectiveness. 

There  is  an  important  difference  between  cast-steel  and  wrought  products.  This  in- 
volves the  presence  of  a  "skin,"  or  thin  layer,  just  below  the  surface  of  a  casting,  where 
scales,  oxides,  and  impurities  are  concentrated.  Also,  this  layer  may  be  chemically  or 
structurally  different  from  the  base  metal.  Therefore,  it  has  to  be  removed  by  machin- 
ing in  a  single  deep  cut,  which  is  achieved  through  reducing  the  cutting  speed  to  half 
of  the  conventionally  recommended  value. 

Gray  cast  iron.  Gray  cast  iron  is  characterized  by  the  presence  of  free  graphite  flakes 
when  its  microstructure  is  examined  under  the  microscope.  This  kind  of  microstructure 
is,  in  fact,  responsible  for  the  superior  properties  possessed  by  gray  cast  iron.  For  in- 
stance, this  dispersion  of  graphite  flakes  acts  as  a  lubricant  during  machining  of  gray 
cast  iron,  thus  eliminating  the  need  for  machining  lubricants  and  coolants.  When  com- 
pared with  any  other  ferrous  cast  alloy,  gray  cast  iron  certainly  possesses  superior 
machinability.  The  presence  of  those  graphite  flakes  is  also  the  reason  for  its  ability  to 
absorb  vibrations.  The  compressive  strength  of  this  iron  is  normally  four  times  its  ten- 
sile strength.  Thus,  gray  cast  iron  has  found  widespread  application  in  machine  tool 
beds  (bases)  and  the  like.  On  the  other  hand,  gray  cast  iron  has  some  disadvantages 
and  limitations,  such  as  its  low  tensile  strength,  brittleness,  and  poor  weldability.  Nev- 
ertheless, gray  cast  iron  has  the  lowest  casting  temperature,  least  shrinkage,  and  the 
best  castability  of  all  cast  ferrous  alloys. 

The  cupola  is  the  most  widely  used  foundry  furnace  for  producing  and  melting 
gray  cast  iron.  The  chemical  composition,  microstructure,  and,  therefore,  the  proper- 
ties of  the  obtained  castings  are  determined  by  the  constituents  of  the  charge  of  the 
cupola  furnace.  Thus,  the  composition  and  properties  of  gray  cast  iron  are  controlled 
by  changing  the  percentages  of  the  charge  constituents  and  also  by  adding  inoculants 
and  alloying  elements.  Commonly  used  inoculants  include  calcium  silicide,  ferrosili- 
con,  and  ferromanganese.  An  inoculant  is  added  to  the  molten  metal  (either  in  the 
cupola  spout  or  ladle)  and  usually  amounts  to  between  0.1  and  0.5  percent  of  the 
molten  iron  by  weight.  It  acts  as  a  deoxidizer  and  also  hinders  the  growth  of  precipi- 
tated graphite  flakes.  It  is  important  for  a  product  designer  to  remember  that  the  prop- 
erties of  a  gray  cast-iron  product  are  also  dependent  upon  the  dimensions  (the 
thicknesses  of  the  walls)  of  that  product  because  the  cooling  rate  is  adversely  affected 
by  the  cross  section  of  the  casting.  Actually,  the  cooling  rate  is  high  for  small  castings 
with  thin  walls,  sometimes  yielding  white  cast  iron.  For  this  reason,  gray  cast  iron 
must  be  specified  by  the  strength  of  critical  cross  sections. 

White  cast  iron.  When  the  molten  cast-iron  alloy  is  rapidly  chilled  after  being 
poured  into  the  mold  cavity,  dissolved  carbon  does  not  have  enough  time  to  precipi- 
tate in  the  form  of  flakes.  Instead,  it  remains  chemically  combined  with  iron  in  the 
form  of  cementite.  This  material  is  primarily  responsible  for  the  whitish  crystalline 
appearance  of  a  fractured  surface  of  white  cast  iron.  Cementite  is  also  responsible  for 


60  3      Casting  and  Foundry  Work 


the  high  hardness,  extreme  brittleness,  and  excellent  wear  resistance  of  this  kind  of 
cast  iron.  Industrial  applications  of  white  cast  iron  involve  components  subjected  to 
abrasion.  Sometimes,  gray  cast  iron  can  be  chilled  to  produce  a  surface  layer  of  white 
cast  iron  in  order  to  combine  the  advantageous  properties  of  the  two  types  of  cast 
iron.  In  this  case,  the  product  metal  is  usually  referred  to  as  chilled  cast  iron. 

Ductile  cast  iron.  Ductile  cast  iron  is  also  called  nodular  cast  iron  and  spheroidal- 
graphite  cast  iron.  It  is  obtained  by  adding  trace  amounts  of  magnesium  to  a  very 
pure  molten  alloy  of  gray  cast  iron  that  has  been  subjected  to  desulfurization.  Some- 
times, a  small  quantity  of  cerium  is  also  added  to  prevent  the  harmful  effects  of  im- 
purities like  aluminum,  titanium,  and  lead.  The  presence  of  magnesium  and  cerium 
causes  the  graphite  to  precipitate  during  solidification  of  the  molten  alloy  in  the  form 
of  small  spheroids,  rather  than  flakes  as  in  the  case  of  gray  cast  iron.  This  mi- 
crostructural  change  results  in  a  marked  increase  in  ductility,  strength,  toughness,  and 
stiffness  of  ductile  iron,  as  compared  with  gray  cast  iron,  because  the  stress  concen- 
tration effect  of  a  flake  is  far  higher  than  that  of  a  spheroid  (remember  what  you 
learned  in  fracture  mechanics).  The  disadvantages  of  ductile  iron,  as  compared  with 
gray  cast  iron,  include  lower  damping  capacity  and  thermal  conductivity.  Ductile  iron 
is  used  for  making  machine  parts  like  axles,  brackets,  levers,  crankshafts,  housings, 
die  pads,  and  die  shoes. 

Compacted-graphite  cast  iron.  Compacted-graphite  (CG)  cast  iron  falls  between  gray 
and  ductile  cast  irons,  both  in  its  microstructure  and  mechanical  properties.  The  free 
graphite  in  this  type  of  iron  takes  the  form  of  short,  blunt,  and  interconnected  flakes. 
The  mechanical  properties  of  CG  cast  iron  are  superior  to  those  of  gray  cast  iron  but 
are  inferior  to  those  of  ductile  cast  iron.  The  thermal  conductivity  and  damping  capac- 
ity of  CG  cast  iron  approach  those  of  gray  cast  iron.  Compacted-graphite  cast  iron  has 
some  application  in  the  manufacture  of  diesel  engines. 

Malleable  cast  iron.  Malleable  cast  iron  is  obtained  by  two-stage  heat  treatment  of 
white  cast  iron  having  an  appropriate  chemical  composition.  The  hard  white  cast 
iron  becomes  malleable  after  the  heat  treatment  due  to  microstructural  changes.  The 
combined  carbon  separates  as  free  graphite,  which  takes  the  form  of  nodules.  Be- 
cause the  raw  material  for  producing  malleable  iron  is  actually  white  cast  iron,  there 
are  always  limitations  on  casting  design.  Large  cross  sections  and  thick  walls  are 
not  permitted  because  it  is  difficult  to  produce  a  white  cast-iron  part  with  these 
geometric  characteristics. 

The  two  basic  types  of  malleable  cast  iron  are  the  pearlitic  and  the  ferritic  (black- 
heart).  Although  the  starting  alloy  for  both  types  is  the  same  (white  cast  iron),  the  heat 
treatment  cycle  and  the  atmosphere  of  the  heat-treating  furnace  are  different  in  each 
case.  Furnaces  with  oxidizing  atmospheres  are  employed  for  producing  pearlitic  mal- 
leable cast  iron,  whereas  furnaces  with  neutral  atmospheres  are  used  for  producing  fer- 
ritic malleable  cast  iron.  When  comparing  the  properties  of  these  two  types,  the  ferritic 
grades  normally  have  higher  ductility  and  better  machinability  but  lower  strength  and 
hardness.  Pearlitic  grades  can,  however,  be  subjected  to  further  surface  hardening 
when  the  depth  of  the  hardened  layer  is  controlled. 


3.3  Classifications  of  Casting  by  Metal  to  Be  Cast 


61 


FIGURE    3.19 

The  heat  treatment 
sequence  for  producing 
malleable  cast  iron 


Temperature  ,, 
(°Cor°F 


(850-950°C) 1700°F 
(800°C) 1400°F 


I 

First  stage 

per  hour 

"/-■ 
I 

-X^               cior-nnH  stage  5°C 

i 

i 

Time  (hours) 

6  hours 

-* Up  to  100  hours +~ 

Figure  3.19  shows  the  heat  treatment  sequence  for  producing  malleable  cast  iron. 
Referred  to  as  the  malleabilizing  cycle,  it  includes  two  stages,  as  shown  in  Figure  3.19. 
In  the  first  stage,  the  casting  is  slowly  heated  to  a  temperature  of  about  1700°F  (950°C) 
and  is  kept  at  that  temperature  for  about  24  hours.  In  the  second  stage,  the  temperature 
is  decreased  very  slowly  at  a  rate  of  5°F  to  9°F  (3°C  to  5°C)  per  hour  from  a  temper- 
ature of  1400°F  (800°C)  to  a  temperature  of  1200°F  (650°C),  where  the  process  ends 
and  the  casting  is  taken  out  of  the  furnace.  The  whole  malleabilizing  cycle  normally 
takes  about  100  hours. 

Malleable  cast  iron  is  usually  selected  when  the  engineering  application  requires 
good  machinability  and  ductility.  Excellent  castability  and  high  toughness  are  other 
properties  that  make  malleable  cast  iron  attractive  as  an  engineering  material.  Typical 
applications  of  malleable  cast  iron  include  flanges,  pipe  fittings,  and  valve  parts  for 
pressure  service  at  elevated  temperatures,  steering-gear  housings,  mounting  brackets, 
and  compressor  crankshafts  and  hubs. 

Alloyed  cast  irons.  Alloying  elements  like  chromium,  nickel,  and  molybdenum  are 
added  to  cast  irons  to  manipulate  the  microstructure  of  the  alloy.  The  goal  is  to  im- 
prove the  mechanical  properties  of  the  casting  and  also  to  impart  some  special  proper- 
ties to  it,  like  resistance  to  wear,  corrosion,  and  heat.  A  typical  example  of  alloyed 
irons  is  the  white  cast  iron  containing  nickel  and  chromium  that  is  used  for  corrosion- 
resistant  (and  abrasion-resistant)  applications  like  water  pump  housings  and  grinding 
balls  (in  a  ball  mill). 

Nonferrous  Metals 

Cast  aluminum  and  its  alloys.  Aluminum  continues  to  gain  wide  industrial  applica- 
tion, especially  in  the  automotive  and  electronics  industries,  because  of  its  distin- 
guished strength-to-weight  ratio  and  its  high  electrical  conductivity.  Alloying  elements 
can  be  added  to  aluminum  to  improve  its  mechanical  properties  and  metallurgical 
characteristics.  Silicon,  magnesium,  zinc,  tin,  and  copper  are  the  elements  most  com- 
monly alloyed  with  aluminum.  In  fact,  most  metallic  elements  can  be  alloyed  with 


62  3      Casting  and  Foundry  Work 


aluminum,  but  commercial  and  industrial  applications  are  limited  to  those  just  men- 
tioned. 

A  real  advantage  of  aluminum  is  that  it  can  be  cast  by  almost  all  casting  processes. 
Nevertheless,  the  common  methods  for  casting  aluminum  include  die  casting,  gravity 
casting  in  sand  and  permanent  molds,  and  investment  casting  (the  lost-foam  process). 

The  presence  of  hydrogen  when  melting  aluminum  always  results  in  unsound 
castings.  Typical  sources  of  hydrogen  are  the  furnace  atmosphere  and  the  charge  metal. 
When  the  furnace  has  a  reducing  atmosphere  because  of  incomplete  combustion  of  the 
fuel,  carbon  monoxide  and  hydrogen  are  generated  and  absorbed  by  the  molten  metal. 
The  presence  of  contaminants  like  moisture,  oil,  or  grease,  which  are  not  chemically 
stable  at  elevated  temperatures,  can  also  liberate  hydrogen.  Unfortunately,  hydrogen  is 
highly  soluble  in  molten  aluminum  but  has  limited  solubility  in  solidified  aluminum. 
Therefore,  any  hydrogen  that  is  absorbed  by  the  molten  metal  is  liberated  or  expelled 
during  solidification,  causing  porosity.  Hydrogen  may  also  react  with  (and  reduce) 
metallic  oxides  to  form  water  vapor,  which  again  causes  porosity.  Thus,  hydrogen  must 
be  completely  removed  from  molten  aluminum  before  casting.  This  is  achieved  by 
using  appropriate  degassers.  Chlorine  and  nitrogen  are  considered  to  be  the  traditional 
degassers  for  aluminum.  Either  of  these  is  blown  through  the  molten  aluminum  to 
eliminate  any  hydrogen.  However,  because  chlorine  is  toxic  and  nitrogen  is  not  that  ef- 
ficient, organic  chloride  fluxing  compounds  (chlorinated  hydrocarbons)  are  added  to 
generate  chlorine  within  the  melt.  They  are  commercially  available  in  different  forms, 
such  as  blocks,  powders,  and  tablets;  the  most  commonly  used  fluxing  degasser  is  per- 
haps hexachlorethane.  Another  source  of  problems  when  casting  aluminum  is  iron, 
which  dissolves  readily  in  molten  aluminum.  Therefore,  care  must  be  taken  to  spray 
(or  cover)  iron  ladles  and  all  iron  surfaces  that  come  into  direct  contact  with  the  molten 
aluminum  with  a  ceramic  coating.  This  extends  the  service  life  of  the  iron  tools  used 
and  also  results  in  sound  castings. 

The  most  important  cast-aluminum  alloys  are  those  containing  silicon,  which 
serves  to  improve  the  castability,  reduce  the  thermal  expansion,  and  increase  the  wear 
resistance  of  aluminum.  Small  additions  of  magnesium  make  these  alloys  heat  treat- 
able, thus  allowing  the  final  properties  of  the  castings  to  be  controlled.  Aluminum- 
silicon  alloys  (with  5  to  13  percent  silicon)  are  used  in  making  automobile  parts  (e.g., 
pistons)  and  aerospace  components. 

Aluminum-copper  alloys  are  characterized  by  their  very  high  tensile-strength-to- 
weight  ratio.  They  are,  therefore,  mainly  used  for  the  manufacture  of  premium-quality 
aerospace  parts.  Nevertheless,  these  alloys  have  poorer  castability  than  the  aluminum- 
silicon  alloys.  Also,  amounts  of  the  copper  constituent  in  excess  of  12  percent  make 
the  alloy  brittle.  Copper  additions  of  up  to  5  percent  are  usually  used  and  result  in  im- 
proved high-temperature  properties  and  machinability. 

Additions  of  magnesium  to  aluminum  result  in  improved  corrosion  resistance  and 
machinability,  higher  strength,  and  attractive  appearance  of  the  casting  when  anodized. 
However,  aluminum-magnesium  alloys  are  generally  difficult  to  cast.  Zinc  is  also  used 
as  an  alloying  element,  and  the  aluminum-zinc  alloys  have  good  machinability  and  mod- 
erately high  strength.  But  these  alloys  are  generally  prone  to  hot  cracking  and  have 
poorer  castability  and  high  shrinkage.  Therefore,  zinc  is  usually  alloyed  with  aluminum 


3.4  Foundry  Furnaces  63 

in  combination  with  other  alloying  elements  and  is  employed  in  such  cases  for  pro- 
moting very  high  strength.  Aluminum-tin  alloys  are  also  in  use.  They  possess  high  load- 
carrying  capacity  and  fatigue  strength  and  are,  therefore,  used  for  making  bearings  and 
bushings. 

Cast  copper  alloys.  The  melting  temperatures  of  cast  copper  alloys  are  far  higher 
than  those  of  aluminum,  zinc,  or  magnesium  alloys.  Cast  copper  alloys  can  be  grouped 
according  to  their  composition  as  follows: 

1.  Pure  copper  and  high-copper  alloys 

2.  Brasses  (alloys  including  zinc  as  the  principal  alloying  element) 

3.  Bronzes  (alloys  including  tin  as  the  principal  alloying  element) 

4.  Nickel  silvers,  including  copper-nickel  alloys  and  copper-nickel-zinc  alloys 

Cast  copper  alloys  are  melted  in  crucible  furnaces,  open-flame  furnaces,  induction 
furnaces,  or  indirect-arc  furnaces.  The  selection  of  a  furnace  depends  upon  the  type  of 
alloy  to  be  melted,  as  well  as  the  purity  and  quantity  required.  In  melting  pure  copper, 
high-copper  alloys,  bronzes,  or  nickel  silver,  precautions  must  be  taken  to  prevent  con- 
tamination of  the  molten  metal  with  hydrogen.  It  is  recommended  that  the  atmosphere 
of  the  furnace  be  slightly  oxidizing  and  also  that  a  covering  flux  be  used.  Prior  to  cast- 
ing, however,  the  molten  metal  should  be  deoxidized  by  adding  phosphorus  in  the  form 
of  a  phosphorous  copper  flux.  On  the  other  hand,  brass  is  usually  not  susceptible  to  hy- 
drogen porosity.  The  problem  associated  with  melting  brass  is  the  vaporization  and  ox- 
idation of  the  zinc.  As  a  remedy,  the  atmosphere  of  the  furnace  should  be  slightly 
reducing.  Also,  a  covering  flux  should  be  used  to  prevent  vaporization  of  the  zinc;  a 
deoxidizing  flux  (like  phosphorous  copper)  is  then  added  immediately  prior  to  pouring. 
The  applications  of  cast-copper  alloys  include  pipe  fitting,  ornaments,  propeller  hubs 
and  blades,  steam  valves,  and  bearings. 

Zinc  alloys.  The  family  of  zinc  alloys  is  characterized  by  low  melting  temperatures. 
Zinc  alloys  also  possess  good  fluidity.  Therefore,  they  can  be  produced  in  thin  sections 
by  submerged-hot-chamber  die  casting.  Alloying  elements  employed  include  alu- 
minum, copper,  and  magnesium. 

Magnesium  alloys.  The  main  characteristic  of  magnesium  is  its  low  density,  which  is 
lower  than  that  of  any  other  commercial  metal.  The  potential  uses  of  magnesium  are 
many  because  it  is  readily  available  as  a  component  of  seawater  and  most  of  its  disad- 
vantages and  limitations  can  be  eliminated  by  alloying.  Magnesium  alloys  usually  are 
cast  in  permanent  molds  or  are  produced  by  hot-chamber  die  casting. 

3.4  FOUNDRY  FURNACES 

Various  furnaces  are  employed  for  smelting  different  ferrous  and  nonferrous  metals  in 
foundry  work.  The  type  of  foundry  furnace  to  be  used  is  determined  by  the  kind  of 
metal  to  be  melted,  the  hourly  output  of  molten  metal  required,  and  the  purity  desired. 
Following  is  a  brief  review  of  each  of  the  commonly  used  foundry  furnaces. 


64 


3      Casting  and  Foundry  Work 


Cupola  Furnaces 

Structure.  The  cupola  is  the  most  widely  used  furnace  for  producing  molten  gray 
cast  iron.  A  sketch  of  a  cupola  furnace  is  given  in  Figure  3.20.  As  can  be  seen,  the 
cupola  is  a  shaft-type  furnace  whose  height  is  three  to  five  times  its  diameter.  It  is 
constructed  of  a  steel  plate  that  is  about  3/8  inch  (10  mm)  thick  and  that  is  internally 
lined  with  refractory  fireclay  bricks.  The  whole  structure  is  erected  on  legs,  or 
columns.  Toward  the  top  of  the  furnace  is  an  opening  through  which  the  charge  is 
fed.  Air,  which  is  needed  for  the  combustion,  is  blown  through  the  tuyeres  located 
about  36  inches  (900  mm)  above  the  bottom  of  the  furnace.  Slightly  above  the  bot- 
tom and  in  the  front  are  a  tap  hole  and  spout  to  allow  molten  cast  iron  to  be  col- 
lected. There  is  also  a  slag  hole  located  at  the  rear  and  above  the  level  of  the  tap  hole 


FIGURE    3.20 

A  cupola  furnace 


Steel  sheet 

Refractory 
lining 


Molten  metal 
Molten-metal 

hole 
— Molten-metal 
passage 


3.4  Foundry  Furnaces 


65 


(because  slag  floats  on  the  surface  of  molten  iron).  The  bottom  of  the  cupola  is 
closed  with  drop  doors  to  dump  residual  coke  or  metal  and  also  to  allow  for  mainte- 
nance and  repair  of  the  furnace  lining. 

Operation.  A  bed  of  molding  sand  is  first  rammed  on  the  bottom  to  a  thickness  of 
about  6  inches  (150  mm)  or  more.  A  bed  of  coke  about  40  inches  (1.0  m)  thick  is  next 
placed  on  the  sand.  The  coke  is  then  ignited,  and  air  is  blown  at  a  lower-than-normal 
rate.  Next,  the  charge  is  fed  into  the  cupola  through  the  charging  door.  Many  factors, 
such  as  the  charge  composition,  affect  the  final  structure  of  the  gray  cast  iron  obtained. 
Nevertheless,  it  can  generally  be  stated  that  the  charge  is  composed  of  25  percent  pig 
iron,  50  percent  gray  cast-iron  scrap,  10  percent  steel  scrap,  12  percent  coke  as  fuel, 
and  3  percent  limestone  as  flux.  These  constituents  form  alternate  layers  of  coke,  lime- 
stone, and  metal.  Sometimes,  ferromanganese  briquettes  and  inoculants  are  added  to 
the  charge  to  control  and  improve  the  structure  of  the  cast  iron  produced. 

Direct  Fuel-Fired  Furnaces 
(Reverberatory  Furnaces) 

The  direct  fuel-fired  furnace,  or  reverberatory  furnace,  is  used  for  the  batch-type  melt- 
ing of  bronze,  brass,  or  malleable  iron.  The  burners  of  the  furnace  are  fired  with  pul- 
verized coal  or  another  liquid  petroleum  product.  Figure  3.21  shows  that  the  roof  of 
the  reverberatory  furnace  reflects  the  flame  onto  the  metal  placed  on  the  hearth,  thus 
heating  the  metal  and  melting  it.  The  gaseous  products  of  combustion  leave  the  furnace 
through  the  flue  duct.  The  internal  surface  of  the  furnace  is  lined  with  fire  bricks,  and 
there  are  charging  and  tap  holes.  When  iron  is  melted,  the  fuel-air  ratio  is  adjusted  to 
produce  a  completely  white  iron  without  free  graphite  flakes  because  they  lower  the 
properties  of  the  resulting  malleable  iron. 

Crucible  (Pot)  Furnaces 

Nonferrous  metals  like  bronzes,  brasses,  aluminum,  and  zinc  alloys  are  usually  melted 
in  a  crucible,  or  pot,  furnace.  Crucible  furnaces  are  fired  by  liquid,  gaseous,  or  pulver- 
ized solid  fuel.  Figure  3.22  shows  that  the  products  of  combustion  in  a  crucible  furnace 


FIGURE    3.21 

A  reverberatory  furnace 


Burner 


Charging 
door 


66 


Casting  and  Foundry  Work 


FIGURE    3.22 

A  crucible  furnace 


Crucible 


Flame 


Refractory 
brick 


do  not  come  in  direct  contact  with  the  molten  metal,  thus  enabling  the  production  of 
quality  castings.  Crucible  furnaces  can  be  stationary  or  tilting.  When  the  stationary 
type  is  employed,  crucibles  are  lifted  out  by  tongs  and  are  then  carried  in  shanks.  On 
the  other  hand,  crucibles  with  long  pouring  lips  are  always  used  with  the  tilting  type. 
Crucibles  are  made  of  either  refractory  material  or  alloy  steels  (containing  25  per- 
cent chromium).  Refractory  crucibles  can  be  of  the  clay-graphite  ceramic-bonded  type 
or  the  silicon-carbide  carbon-bonded  type.  The  first  type  is  cheaper,  while  the  second 
one  is  more  popular  in  industry.  Ceramic  crucibles  are  used  when  melting  aluminum, 
bronze,  or  gray  cast  iron,  whereas  brasses  are  melted  in  alloy  steel  crucibles.  Different 
alloys  must  not  be  melted  in  the  same  crucible  to  avoid  contamination  of  the  molten 
metal. 

Electric  Furnaces 

An  electric  furnace  is  usually  used  when  there  is  a  need  to  prevent  the  loss  of  any  con- 
stituent element  from  the  alloy  and  when  high  purity  and  consistency  of  casting  qual- 
ity are  required.  An  electric  furnace  is  also  employed  when  melting  high-temperature 
alloys.  In  all  types  of  electric  furnaces,  whether  they  are  electric-arc,  resistance,  or  in- 
duction furnaces,  the  electric  energy  is  converted  into  heat. 

Electric-arc  furnace.  The  electric-arc  furnace  is  the  most  commonly  used  type  of 
electric  furnace.  Figure  3.23  is  a  sketch  of  an  electric-arc  furnace.  The  heat  generated 
by  an  electric  arc  is  transferred  by  direct  radiation  or  by  reflected  radiation  off  the  in- 


FIGURE    3.23 

An  electric-arc  furnace 


Pouring 
spout 


Gear  system  (for  rotating 
rum  at  an  adequate 
e  for  pouring  the 
molten  metal) 


3.4  Foundry  Furnaces 


67 


FIGURE    3.24 

An  electric-resistance 
furnace 


Refractory 

lining 

Molten 
metal 
Insulating 

material 


Pouring 
spout 


ternal  lining  of  the  furnace.  The  electric  arc  is  generated  about  midway  between  two 
graphite  electrodes.  In  order  to  control  the  gap  between  the  two  electrodes  and,  ac- 
cordingly, control  the  intensity  of  heat,  one  electrode  is  made  stationary  and  the  other 
one  movable.  Electric-arc  furnaces  are  used  mainly  for  melting  steels  and,  to  a  lesser 
extent,  gray  cast  iron  and  some  nonferrous  metals. 

Resistance  furnace.  The  resistance  furnace  is  employed  mainly  for  melting  alu- 
minum and  its  alloys.  Figure  3.24  indicates  the  basic  features  of  a  typical  resistance 
furnace.  The  solid  metal  is  placed  on  each  of  the  two  inclined  hearths  and  is  subjected 
to  heat  radiation  from  the  electric-resistance  coils  located  above  it.  When  the  metal 
melts,  it  flows  down  into  a  reservoir.  The  molten  metal  can  be  poured  out  through  the 
spout  by  tilting  the  whole  furnace. 

Induction  furnace.  The  induction  furnace  has  many  advantages,  including  evenly  dis- 
tributed temperatures  within  the  molten  metal,  flexibility,  and  the  possibility  of  con- 
trolling the  atmosphere  of  the  furnace.  In  addition,  the  motor  effect  of  the 
electromagnetic  forces  helps  to  stir  the  molten  metal,  thus  producing  more  homoge- 
neous composition.  Induction  furnaces  are  used  to  melt  steel  and  aluminum  alloys. 
Figure  3.25  shows  the  construction  of  a  typical  induction  furnace.  It  basically  involves 
an  electric-induction  coil  that  is  built  into  the  walls  of  the  furnace.  An  alternating  cur- 
rent in  the  coil  induces  current  in  any  metallic  object  that  obstructs  the  electromagnetic 


FIGURE    3.25 

An  electric-induction 
furnace 


Cover 


Pouring 
spout 


Molten  metal 

(under  stirring 

action) 


Electric-induction 
coil  (copper  tubing) 


68  3      Casting  and  Foundry  Work 


flux.  Furnaces  of  both  high-  and  low-frequency  current  are  successfully  used  in  indus- 
try to  induce  alternating  current  in  solid  metal  to  melt  it. 


ASTING  DEFECTS  AND 
DESIGN  CONSIDERATIONS 


Common  Defects  in  Castings 

In  order  to  obtain  a  sound  casting,  it  is  necessary  to  control  adequately  the  various  fac- 
tors affecting  the  casting  process.  Casting  and  pattern  designs,  molding  procedure,  and 
melting  and  pouring  of  molten  metal  are  among  the  factors  affecting  the  soundness  of 
a  casting.  Following  is  a  survey  of  the  commonly  experienced  defects  in  castings. 

Hot  tears.  Hot  tears  can  appear  on  the  surface  or  through  cracks  that  initiate  during 
cooling  of  the  casting.  They  usually  are  in  locations  where  the  metal  is  restrained  from 
shrinking  freely,  such  as  a  thin  wall  connecting  two  heavy  sections. 

Cold  shut.  A  cold  shut  is  actually  a  surface  of  separation  within  the  casting.  It  is  be- 
lieved to  be  caused  by  two  "relatively  cold"  streams  of  molten  metal  meeting  each 
other  at  that  surface. 

Sand  wash.  A  sand  wash  can  be  described  as  rough,  irregular  surfaces  (hills  and  val- 
leys) of  the  casting  that  result  from  erosion  of  the  sand  mold.  This  erosion  is,  in  turn, 
caused  by  the  metal  flow. 

Sand  blow.  A  sand  blow  is  actually  a  surface  cavity  that  takes  the  form  of  a  very 
smooth  depression.  It  can  be  caused  by  insufficient  venting,  lack  of  permeability,  or  a 
high  percentage  of  humidity  in  the  molding  sand. 

Scab.  A  scab  is  a  rough  "swollen"  location  in  the  casting  that  has  some  sand  embed- 
ded in  it.  Such  a  defect  is  usually  encountered  when  the  molding  sand  is  too  fine  or  too 
heavily  rammed. 

Shrinkage  porosity  (or  cavity).  A  shrinkage  porosity  is  a  microscopic  or  macroscopic 
hole  formed  by  the  shrinkage  of  spots  of  molten  metal  that  are  encapsulated  by  solid- 
ified metal.  It  is  usually  caused  by  poor  design  of  the  casting. 

Hard  spots.  Hard  spots  are  hard,  difficult-to-machine  areas  that  can  occur  at  different 
locations. 

Deviation  of  the  chemical  composition  from  the  desired  one.  Deviation  may  be  due 
to  the  loss  of  a  constituent  element  (or  elements)  during  the  melting  operation.  It  may 
also  be  caused  by  contamination  of  the  molten  metal. 

Design  Considerations 

A  product  designer  who  selects  casting  as  the  primary  manufacturing  process  should 
make  a  design  not  only  to  serve  the  function  (by  being  capable  of  withstanding  the 
loads  and  the  environmental  conditions  to  which  it  is  going  to  be  subjected  during  its 


3.5  Casting  Defects  and  Design  Considerations 


69 


service  life)  but  also  to  facilitate  or  favor  the  casting  process.  Following  are  some  de- 
sign considerations  and  guidelines. 

Promote  directional  solidification.  When  designing  the  mold,  be  sure  that  the  risers 
are  properly  dimensioned  and  located  to  promote  directional  solidification  of  the  cast- 
ing toward  the  risers.  In  other  words,  the  presence  of  large  sections  or  heat  masses  in 
locations  distant  from  the  risers  should  be  avoided,  and  good  rising  practice  as  previ- 
ously discussed  should  be  followed.  Use  can  also  be  made  of  chills  to  promote  direc- 
tional solidification.  Failure  to  do  so  may  result  in  shrinkage  cavities  (porosity)  or 
cracks  in  those  large  sections  distant  from  the  risers.  It  is  also  very  important  to  re- 
member that  a  riser  will  not  feed  a  heavy  section  through  a  lighter  section. 

Ensure  easy  pattern  drawing.  Make  sure  that  the  pattern  can  easily  be  withdrawn  from 
the  nonpermanent  mold  (this  does  not  apply  to  investment  casting).  This  can  be 
achieved  through  rational  selection  of  the  parting  line  as  well  as  by  providing  appropri- 
ate pattern  draft  wherever  needed.  In  addition,  undercuts  or  protruding  bosses  (espe- 
cially if  their  axes  do  not  fall  within  the  parting  plane)  and  the  like  should  be  avoided. 
Nevertheless,  remember  that  undercuts  can  be  obtained,  if  necessary,  by  using  cores. 

Avoid  the  shortcomings  of  columnar  solidification.  Dendrites  often  start  to  form  on 
the  cold  surface  of  a  mold  and  then  grow  to  form  a  columnar  casting  structure.  This 
almost  always  results  in  planes  of  weakness  at  sharp  corners,  as  illustrated  in  Fig- 
ure 3.26a.  Therefore,  rounding  the  edges  is  a  must  for  eliminating  the  development  of 
planes  of  weakness,  as  shown  in  Figure  3.26b.  Rounded  edges  are  also  essential  for 
smooth  laminar  flow  of  the  molten  metal. 

Avoid  hot  spots.  Certain  shapes,  because  of  their  effect  on  the  rate  of  heat  dissipation 
during  solidification,  tend  to  promote  the  formation  of  shrinkage  cavities.  This  is  al- 
ways the  case  at  any  particular  location  where  the  rate  of  solidification  is  slower  than 
that  at  the  surrounding  regions  of  the  casting.  The  rate  of  solidification  (and  the  rate 
of  heat  dissipation  to  start  with)  is  slower  at  locations  having  a  low  ratio  of  surface 
area  to  volume.  Such  locations  are  usually  referred  to  as  hot  spots  in  foundry  work. 
Unless  precautions  are  taken  during  the  design  phase,  hot  spots  and,  consequently, 
shrinkage  cavities  are  likely  to  occur  at  the  L,  T,  V,  Y,  and  +  junctions,  as  illustrated  in 
Figure  3.27a.  Shrinkage  cavities  can  be  avoided  by  modifying  the  design,  as  shown  in 


FIGURE    3.26 

Columnar  solidification 
and  planes  of 
weakness: (a)  poor 
design  (sharp  corner); 
(b)  rounded  edges  to 
eliminate  planes  of 
weakness 


(a) 


(b) 


70 


3      Casting  and  Foundry  Work 


FIGURE    3.27 

Hot  spots:  (a)  poor 
design,  yielding  hot 
spots;  (b)  better 
design,  eliminating  hot 
spots 


rcn 


<2X         Cored 
holes 


(b) 


Figure  3.27b.  Also,  it  is  always  advisable  to  avoid  abrupt  changes  in  sections  and  to 
use  taper  (i.e.,  make  the  change  gradual),  together  with  generous  radii,  to  join  thin  to 
heavy  sections,  as  shown  in  Figure  3.28. 

Avoid  the  causes  of  hot  tears.  Hot  tears  are  casting  defects  caused  by  tensile  stresses 
as  a  result  of  restraining  a  part  of  the  casting.  Figure  3.29a  and  b  shows  locations  where 
hot  tears  can  occur  and  a  recommended  design  that  would  eliminate  their  formation. 

Distribute  the  masses  of  a  section  to  save  material.  Cast  metals  are  generally 
weaker  in  tension  in  comparison  with  their  compressive  strengths.  Nonetheless,  the 
casting  process  offers  the  designer  the  flexibility  of  distributing  the  masses  of  a  section 
with  a  freedom  not  readily  available  when  other  manufacturing  processes  are  em- 
ployed. Therefore,  when  preparing  a  design  of  a  casting,  try  to  distribute  masses  in 
such  a  manner  as  to  lower  the  magnitude  of  tensile  stresses  in  highly  loaded  areas  of 
the  cross  section  and  to  reduce  material  in  lightly  loaded  areas.  As  can  be  seen  in  Fig- 
ure 3.30,  a  T  section  or  an  I  beam  is  more  advantageous  than  just  a  round  or  square 
one  when  designing  a  beam  that  is  to  be  subjected  to  bending. 

Avoid  thicknesses  lower  than  the  recommended  minimum  section  thickness.    The 

minimum  thickness  to  which  a  section  of  a  casting  can  be  designed  depends  upon  such 
factors  as  the  material,  the  size,  and  the  shape  of  the  casting  as  well  as  the  specific 


FIGURE    3.28 

Avoiding  abrupt 
changes  in  sections 


@=<o) 


Poor 


Poor 


O 


o 


Better 


Better 


3.5  Casting  Defects  and  Design  Considerations 


71 


FIGURE    3.29 

Hot  tears:  (a)  a  casting 
design  that  promotes 
hot  tears;  (b) 
recommended  design 
to  eliminate  hot  tears 


(a) 


(b) 


casting  process  employed  (i.e.,  sand  casting,  die  casting,  etc.).  In  other  words,  strength 
and  rigidity  calculations  may  prove  a  thin  section  to  be  sufficient,  but  casting  consid- 
erations may  require  adopting  a  higher  value  for  the  thickness  so  that  the  cast  sections 
will  fill  out  completely.  This  is  a  consequence  of  the  fact  that  a  molten  metal  cools 
very  rapidly  as  it  enters  the  mold  and  may  become  too  cold  to  fill  a  thin  section  far 
from  the  gate.  A  minimum  thickness  of  0.25  inch  (6  mm)  is  suggested  for  design  use 
when  conventional  steel  casting  techniques  are  employed,  but  wall  thicknesses  of 
0.060  inch  (1.5  mm)  are  quite  common  for  investment  castings.  Figure  3.31  indicates 
the  relationship  between  the  minimum  thickness  of  a  section  and  its  largest  dimension. 
It  should  be  pointed  out  that  for  a  given  thickness,  steel  flows  best  in  a  narrow  rather 
than  in  a  wide  web.  For  cast-iron  and  nonferrous  castings,  the  recommended  values  for 
minimum  thicknesses  are  much  lower  than  those  for  steel  castings  having  the  same 
shape  and  dimensions. 

Strive  to  make  small  projections  in  a  large  casting  separate.  As  can  be  seen  in  Fig- 
ure 3.32a,  a  small  projection  may  be  subjected  to  more  accidental  knocks  than  a  large 


FIGURE    3.30 

Distribution  of  masses 
to  reduce  weight 


nbending 


Beam 


cr7 


S 


T  section 


Stress  distribution 


I  beam 


Stress  distribution 


Very  low 
stress  area 
(material  not 
fully  utilized) 


Square  bar 


Stress  distribution 


72 


3      Casting  and  Foundry  Work 


FIGURE    3.31 

0.375 

Minimum  thickness  of 

0  25 

cast  steel  sections  as 

a  function  of  their 

</> 

0.125 

largest  dimension 
(Adapted  from  Steel 
Castings  Handbook, 

<0 

c 

o 

E 

0 
0.750 

5th  ed.  Rocky  River, 
Ohio:  Steel  Founders 

E 
c 
2 

0.500 

Society  of  America, 
1980) 

0.250 
0 

_  ^^^^-^ 

Length  of  section  (cm) 

100 

200 

i 

300 
i 

400        500        600        700 
i             i             i             i 

- 

- 

I            1 

i         i 

i 

E 
E, 

30  » 


20  £ 

E 

E 
10  I 


50  100  150  200 

Length  of  section  (in.) 


250 


300 


casting,  and  if  it  gets  broken,  the  whole  casting  will  be  scrapped.  It  is,  therefore,  highly 
recommended  to  make  the  small  projection  separate  and  attach  it  to  the  large  casting 
by  an  appropriate  mechanical  joining  method,  as  shown  in  Figure  3.32b. 

Strive  to  restrict  machined  surfaces.  Whereas  some  castings  are  used  in  their  en- 
tirely as-cast  condition,  some  others  may  require  one  or  more  machining  operations.  It 
is  the  task  of  the  designer  to  ensure  that  machining  is  performed  only  on  areas  where 
it  is  absolutely  necessary.  An  example  of  cases  where  the  machining  needed  involves 
bearing  surfaces  is  shown  in  Figure  3.33. 

Use  reinforcement  ribs  to  improve  the  rigidity  of  thin,  large  webs.  A  common  use  of 
brackets  or  reinforcement  ribs  is  to  provide  rigidity  to  thin,  large  webs  (or  the  like)  as 
an  alternative  to  increasing  the  thickness  of  the  webs.  The  ribs  should  be  as  thin  as 
possible  (i.e.,  minimum  permissible  thickness)  and  should  also  be  staggered,  as  shown 
in  Figure  3.34.  Always  remember  that  parabolic  ribs  are  better  than  straight  ribs  in 
terms  of  economy  and  uniformity  of  stress. 

Consider  the  use  of  cast-weld  construction  to  eliminate  costly  cored  design.  The  de- 
sign of  some  products  necessitates  the  use  of  complicated  steel- wire-reinforced  cores 
that  are  difficult  to  reach  and  remove  after  casting,  thus  leaving  the  surfaces  unclean. 
An  example,  a  steam  ring,  is  shown  in  Figure  3.35a.  The  alternative  design  would  be 
to  employ  a  simple  cut  plate  that  is  welded  into  the  casting  to  produce  the  cast-weld 
construction  shown  in  Figure  3.35b. 


3.6  CLEANING,  TESTING,  AND 
INSPECTION  OF  CASTINGS 

Cleaning 


The  cleaning  process  involves  the  removal  of  the  molding  sand  adhering  to  a  casting. 
It  also  includes  the  elimination  of  gates,  runners,  and  risers.  Generally,  surface  clean- 
ing can  be  carried  out  in  rotary  separators  or  by  employing  sand-blasting  and/or  metal- 


3.6  Cleaning,  Testing,  and  Inspection  of  Castings 


73 


FIGURE    3.32 

Large  casting  with  a 
small  projection:  (a)  as 
an  integral  part;  (b)  two 
separate  parts 


Small  projection 


lie  shot-blasting  machines.  The  latter  two  machines  use  sand  particles  or  shots  travel- 
ing at  high  velocities  onto  the  surface  of  the  casting  to  loosen  and  remove  the  adher- 
ing sand.  As  you  may  expect,  these  machines  are  particularly  suitable  when  cleaning 
medium  and  heavy  castings.  On  the  other  hand,  rotary  separators  are  advantageous  for 
cleaning  light  castings.  A  separator  is  actually  a  long,  large-diameter  drum  that  ro- 
tates around  its  horizontal  axis  into  which  the  castings  are  loaded  together  with  jack 


74 


3      Casting  and  Foundry  Work 


FIGURE    3.33 

Restriction  of  surfaces 
to  be  machined 


FIGURE    3.34 

Use  of  reinforcement 
ribs 


1  in. 
(25  mm) 


FIGURE    3.35 

The  design  of  a  steam 
ring:  (a)  cast 
construction;  (b)  cast- 
weld  construction 


7777/AW////////A'. 


/////////////////////// / TT7 
(a) 


7777/7^) /////// 77^777. _ 


>1\\\\\\\\\\\\\\\\\\\\\\\\\ 


Plate  welded 
(b) 


3.7  Castability  (Fluidity)  75 

stars  made  of  white  cast  iron.  A  further  advantage  of  rotary  separators  is  that  they  au- 
tomatically break  off  gate-and-runner  systems  and,  often,  risers. 

Testing  and  Inspection 

Like  any  other  manufactured  parts,  castings  must  be  subjected  to  thorough  quality  con- 
trol in  order  to  separate  defective  products  and  to  reduce  the  percentage  of  rejects 
through  identifying  the  defects  and  tracing  their  sources.  Following  are  some  of  the 
commonly  used  tests  and  inspection  methods. 

Testing  of  the  mechanical  properties  of  the  casting.  Standard  tension  and  hardness 
tests  are  carried  out  to  determine  the  mechanical  properties  of  the  metal  of  the  casting 
in  order  to  make  sure  that  they  conform  to  the  specifications. 

Inspection  of  the  dimensions.  Dimensions  must  fall  within  the  specified  limits. 
Therefore,  measuring  tools  and  different  kinds  of  gages  (e.g.,  snap,  progressive,  plug, 
template)  are  used  to  check  that  the  dimensions  conform  to  the  blueprint. 

Visual  examination.  Visual  inspection  is  used  to  reveal  only  very  clear  defects.  How- 
ever, it  is  still  commonly  used  in  foundries. 

Hydraulic  leak  testing.  The  hydraulic  leak  test  is  used  to  detect  microscopic  shrink- 
age porosity.  Various  penetrants  and  testing  methods  are  now  available.  Details  are 
given  in  the  American  Society  for  Testing  and  Materials  (ASTM)  standards,  designa- 
tion E165. 

Nondestructive  testing.  There  are  several  nondestructive  testing  methods  that  detect 
microscopic  and  hair  cracks.  They  involve  ultrasonic  testing,  magnetic  particle  inspec- 
tion, eddy  current  testing,  and  radiography. 

Testing  for  metal  composition.  Several  methods  are  employed  to  determine  the  chem- 
ical composition  accurately  and  to  assure  product  quality.  The  classical  method  used  to 
be  "wet  analysis"  (i.e.,  employing  acids  and  reagents  in  accurate  chemical  analysis). 
However,  because  this  method  is  time-consuming,  it  is  being  replaced  by  methods  like 
emission  spectroscropy,  X-ray  fluorescence,  and  atomic  absorption  spectroscopy. 


CASTABILITY  (FLUIDITY) 


The  ability  of  the  molten  metal  to  flow  easily  without  premature  solidification  is  a 
major  factor  in  determining  the  proper  filling  of  the  mold  cavity.  This  important  prop- 
erty is  referred  to  as  castability  or,  more  commonly,  fluidity.  The  higher  the  fluidity  of 
a  molten  metal,  the  easier  it  is  for  that  molten  metal  to  fill  thin  grooves  in  the  mold  and 
exactly  reproduce  the  shape  of  the  mold  cavity,  thereby  successfully  producing  cast- 
ings with  thinner  sections.  Poor  fluidity  leads  to  casting  defects  such  as  incomplete  fill- 
ing or  misruns,  especially  in  the  thinner  sections  of  a  casting.  Because  fluidity  is 
dependent  mainly  upon  the  viscosity  of  the  molten  metal,  it  is  clear  that  higher  tem- 
peratures improve  the  fluidity  of  molten  metal  and  alloys,  whereas  the  presence  of  im- 
purities and  nonmetallic  inclusions  adversely  affects  it. 


76 


3      Casting  and  Foundry  Work 


FIGURE    3.36 

Details  of  the  test  for 
measuring  fluidity 


0.3  inch 

n 


«-i*A7I[a3 


inch 


Magnified 
section  A-A 


Several  attempts  have  been  made  to  quantify  and  measure  the  fluidity  of  metals. 
A  commonly  used  standard  test  involves  pouring  the  molten  metal  into  a  basin  so  that 
it  flows  along  a  spiral  channel  of  a  particular  cross  section,  as  shown  in  Figure  3.36. 
Both  the  basin  and  the  channel  are  molded  in  sand,  and  the  fluidity  value  is  indicated 
by  the  distance  traveled  by  the  molten  metal  before  it  solidifies  in  the  spiral  channel. 


Review  Questions 


1 


1.  What  is  meant  by  the  word  casting  ? 

2.  What  are  the  constituents  of  green  molding 
sand? 

3.  List  some  of  the  important  properties  that  green 
sand  must  possess. 

4.  What  is  a  flask?  What  is  its  function?  List  the 
parts  that  form  a  flask. 

5.  Explain  the  meaning  of  the  word  pattern. 

6.  List  some  of  the  materials  used  in  making  pat- 
terns. 

7.  List  the  different  types  of  permanent  patterns 
used  in  foundries. 


8.  What  are  the  different  pattern  allowances?  Dis- 
cuss the  function  of  each. 

9.  What  are  cores?  How  are  they  made? 

10.  What  is  meant  by  a  gating  system  ?  What  func- 
tions does  it  serve? 

11.  What  are  the  components  of  a  gating  system? 

12.  What  are  risers?  What  function  do  they  serve? 

13.  List  the  various  green  sand  properties  and  dis- 
cuss each  briefly. 

14.  Why  should  weights  be  located  on  the  cope  in 
pit  molding? 


Chapter  3  Review  Questions 


77 


15.  List  the  various  molding  machines  and  discuss 
the  operation  of  each  briefly. 

16.  Explain  sand  conditioning  and  how  it  is  done. 

17.  What  advantages  does  dry  sand  molding  have 
over  green  sand  molding? 

18.  When  are  cement-bonded  sand  molds  recom- 
mended? 

19.  What  is  the  main  advantage  of  the  carbon  diox- 
ide process  for  molding? 

20.  What  metals  can  be  cast  in  plaster  molds? 

21.  When  are  loam  molds  used? 

22.  Describe  shell  molding.  What  are  its  advan- 
tages? 

23.  When  are  ceramic  molds  recommended? 

24.  Explain  investment  casting  and  why  it  is  some- 
times called  the  lost-wax  process. 

25.  Name  a  metal  that  should  be  cast  in  a  graphite 
mold. 

26.  What  are  the  advantages  of  employing  perma- 
nent molds?  Why? 

27.  Can  molten  metals  be  cast  directly  into  cavities 
of  cold  permanent  molds?  Why? 

28.  What  is  the  main  difference  between  the  hot- 
chamber  and  the  cold-chamber  methods  of  die 
casting? 

29.  List  some  metals  that  you  think  can  be  cast  by 
the  hot-chamber  method.  Justify  your  answer. 

30.  List  some  metals  that  you  think  can  be  cast  by 
the  cold-chamber  method.  Justify  your  answer. 

31.  What  are  the  types  of  centrifugal  casting? 

32.  Differentiate  between  the  different  types  of 
centrifugal  casting  and  discuss  the  advantages 
and  shortcomings  of  each  type. 

33.  What  are  the  products  that  can  be  manufactured 
by  continuous  casting? 

34.  What  does  the  continuous  casting  process  in- 
volve? 

35.  Discuss  some  advantages  of  the  continuous 
casting  process. 


36.  What  does  the  V-process  involve? 

37.  List  some  of  the  merits  and  advantages  of  the 
V-process. 

38.  Discuss  some  of  the  problems  encountered  in 
casting  steels. 

39.  What  precautions  should  be  taken  to  eliminate 
the  problems  in  casting  steels? 

40.  What  is  gray  cast  iron? 

41.  Discuss  some  of  the  properties  that  make  gray 
cast  iron  attractive  for  some  engineering  appli- 
cations. 

42.  Why  are  inoculants  added  to  gray  cast  iron? 

43.  Differentiate  between  gray  cast  iron  and  white 
cast  iron. 

44.  What  is  meant  by  compacted-graphite  cast  iron. 

45.  What  is  ductile  cast  iron?  How  can  it  be  ob- 
tained? 

46.  What  is  malleable  cast  iron?  How  can  it  be 
obtained?  What  are  the  limitations  on  produc- 
ing it? 

47.  List  some  alloying  elements  that  are  added  to 
cast  iron.  List  some  applications  for  alloyed 
cast  iron. 

48.  What  are  the  problems  caused  by  hydrogen 
when  melting  and  casting  aluminum  and  how 
can  these  problems  be  eliminated? 

49.  What  are  the  sources  of  hydrogen  when  melting 
aluminum? 

50.  List  some  cast  aluminum  alloys  and  discuss 
their  applications. 

51.  How  are  cast  copper  alloys  classified? 

52.  What  is  meant  by  a  deoxidizer?  Give  an  ex- 
ample. 

53.  List  some  of  the  characteristics  and  applica- 
tions of  cast  zinc  alloys. 

54.  List  some  of  the  characteristics  and  applica- 
tions of  cast  magnesium  alloys. 

55.  For  what  purpose  is  the  cupola  furnace  used? 

56.  Describe  briefly  the  operation  and  charge  of  the 
cupola  furnace. 


78 


3      Casting  and  Foundry  Work 


57. 


58 


For  what  purpose  is  the  reverberatory  furnace 
used? 


List  some  of  the  metals  that  can  be  melted  in 
crucible  furnaces. 

59.  What  are  the  main  differences  in  construction 
between  the  stationary  and  the  tilting  crucible 
furnaces? 

60.  List  the  different  types  of  electric  furnaces  and 
mention  the  principles  of  operation  in  each  case. 


61.  List  the  main  advantages  and  applications  of 
electric  furnaces. 

62.  List  some  of  the  common  defects  of  castings 
and  discuss  the  possible  causes  of  each  defect. 

63.  List  and  discuss  the  main  design  considerations 
for  castings. 

64.  List  and  discuss  the  various  testing  and  inspec- 
tion methods  used  for  the  quality  control  of 
castings. 


Design  Example 


_z 


PROBLEM 

Your  company  has  received  an  order  to  manufacture  wrenches  for  loosening  and  tight- 
ening nuts  and  bolts  of  large  machines.  The  plant  of  the  company  involves  a  foundry 
and  a  machining  workshop  with  a  few  basic  machine  tools.  Here  are  the  details  of  the 
order: 


Lot  size: 
Nut  size: 
Required  torque: 


500  wrenches 

2  inches  (50  mm) 

about  20  lb  ft  (27.12  N-m) 


You  are  required  to  provide  a  design  and  a  production  plan  (see  the  explanation  of  the 
word  design  in  the  design  projects  section  that  appears  later). 

Solution 

Before  we  start  solving  this  design  problem,  we  should  make  some  assumptions.  For 
instance,  consider  the  force  that  can  be  generated  by  the  ordinary  human  hand.  It  will 
allow  us  to  determine  the  length  of  the  wrench  using  the  following  equation: 

T=Fxt 

where:  T  is  the  torque 
F  is  the  force 
(,  is  the  length 

As  can  be  seen  from  the  equation,  a  low  value  of  F  would  make  the  length  large  and 
thus  make  the  handling  of  the  wrench  impractical  because  of  the  weight.  On  the  other 
hand,  a  high  value  of  F  is  not  practical  and  may  not  be  generated  by  an  ordinary  per- 
son. Let  us  take  F  =  15  pounds.  Therefore, 


Chapter  3  Design  Example  79 


^  =  T7=  1-33  feet 

Apparently,  the  force  acts  at  the  middle  of  the  fist,  and  we  have  to  add  a  couple  of 
inches  for  proper  holding: 

length  of  wrench  =  1 8  inches 

Let  us  now  design  the  section  where  the  maximum  bending  moment  occurs.  You 
can  assume  some  dimensions  and  determine  the  stress,  which  will  serve  as  a  guide  in 
selecting  material.  Take  the  section  as  shown  in  Figure  3.37a.  The  moment  of  inertia 
of  the  section  is 


/  =  -j^(0.25)(0.75)3  +  2 


— (0.375)(0.25)3  +  0.375  x  0.25  x  0.5 


=  0.008789  +  0.00098  +  0.046875 
=  0.056655  in.4 

Note  that  the  minimum  thickness  for  steel  casting  was  adhered  to.  Now,  determine  the 
stress: 

20  x  12  x  1.25      .,..  „  „    2 

max.  stress  = =  2648  lb/in. 

2  x  0.056655 

That  value  is  very  low,  and  we  should  try  to  reduce  the  section  and  save  material.  It  is 
always  a  good  idea  to  make  use  of  spreadsheets  to  change  the  dimensions  and  get  the 
stresses  acting  in  each  case.  Now,  take  the  section  as  shown  in  Figure  3.37b: 


/  =  ^(0.25)(0.5)3  +  2 


— (0.375)(0.25)3  +  0.375  x  0.25  x  0.3752 


=  0.002604  +  0.00098  +  0.026367 
=  0.038771188  in.4 

20  x  12  1.25      ,0,01ur    2 

max.  stress  =  — — —  x  — ——  =  3868  lb/in. 

0.038771188         2 

As  can  be  seen,  we  took  the  minimum  thickness  to  be  0.25  inch,  which  is  the  recom- 
mended value  for  conventional  castings  of  steels. 

The  material  should  be  low-carbon  steel  having  0.25  percent  carbon  in  order  to 
possess  enough  ductility.  Also,  the  steel  should  be  thoroughly  killed.  A  recommended 
material  is  ASTM  A27-77,  grade  U60-30,  which  has  a  yield  strength  of  30  ksi.  When 
taking  a  factor  of  safety  of  4,  the  allowable  stress  would  be  7500  lb/in. ,  which  is 
higher  than  the  obtained  value  of  the  working  stress. 

Now,  in  order  to  calculate  the  thickness  of  the  wrench,  let  us  calculate  the  bearing 
stress  on  the  nut.  A  reasonable  estimate  of  the  force  on  the  surface  of  the  nut  is 

20  x  12 

=  320  pounds 

0.75  F 


80 


3      Casting  and  Foundry  Work 


FIGURE    3.37 

Cross  section  of  the 
wrench:  (a)  first 
attempt;  (b)  second 
attempt 


0.25  inch 


0.75  inch 


0.25  inch 


0.25  inch 


0.5  inch 


0.25  inch 


This  is  based  on  the  assumption  that  the  torque  is  replaced  by  two  opposite  forces  hav- 
ing a  displacement  of  0.75  inch  between  the  lines  of  action.  Thus, 


bearing  stress  = 


320 


=  7500 


0.75  x  t 
t  =  0.056  inch 

Take  it  as  0.5  inch  to  facilitate  casting  the  part. 

Because  all  dimensions  are  known,  a  detailed  design  can  be  prepared,  as  shown  in 
Figure  3.38.  Notice  the  surface  finish  marks  indicating  the  surfaces  to  be  machined. 


FIGURE    3.38 

A  wrench  manufactured  by  casting 


f?=  1.0  inch 


1.4  inch 


'ncn 


0.5  inch 


:c 


Djo. 


375  inch 


Chapter  3  Design  Projects  81 

As  previously  mentioned,  conventional  sand  casting  is  to  be  employed,  using  a 
cope-and-drag  pattern  plate  to  cast  two  wrenches  per  flask.  We  can  use  a  single  down 
sprue  to  feed  the  narrow  end  of  the  wrench  and  a  riser  at  the  other  end.  The  parting 
line  will  pass  through  the  web  of  the  /  section. 


Design  Projects 


Whenever  the  word  design  is  mentioned  hereafter,  you  should  provide,  at  least,  the 
following: 

•  Two  neatly  dimensioned  graphical  projections  of  the  product  (i.e.,  a  blueprint  ready 
to  be  released  to  the  workshop  for  actual  production),  including  fits  (if  applicable), 
tolerances,  surface  finish  marks,  and  so  on 

•  Material  selection  with  rational  justification 

•  Selection  of  the  specific  manufacturing  processes  required,  as  well  as  their  se- 
quence in  detail 

•  Simple  but  necessary  calculations  to  check  the  stresses  at  the  critical  sections 

1.  Design  a  bracket  for  a  screw  C-clamp  that  has  the  following  characteristics: 

Maximum  clamping  force:  22  pounds  (100  N) 

Clamping  gap:  3  inches  (7.5  cm) 
Distance  between  centerline  of  screw 

and  inner  surface  of  bracket:  2  inches  (5  cm) 

Root  diameter  of  screw:  0.25  inch  (6  mm) 

Assume  that  manufacturing  is  by  casting  and  that  production  volume  is  4000 
pieces. 

2.  Design  a  flat  pulley.  Its  outer  diameter  is  36  inches  (90  cm),  and  it  is  to  be 
mounted  on  a  shaft  that  is  2x/i  inches  (6.25  cm)  in  diameter.  Its  width  is  10  inches 
(25  cm),  and  it  has  to  transmit  a  torque  of  3000  lb  ft  (4000  Nm).  Assume  that  500 
pieces  are  required.  Will  the  design  change  if  only  3  pieces  are  required? 

3.  A  connecting  lever  has  two  short  bosses,  each  at  one  of  its  ends  and  each  with  a 
vertical  hole  that  is  3/4  inch  (19  mm)  in  diameter.  The  lever  is  straight,  and  the 
horizontal  distance  between  the  centers  of  the  holes  is  8  inches  (200  mm).  The 
lever  during  functioning  is  subjected  to  a  bending  moment  of  50  lb  ft  (67.8  Nm) 
that  acts  in  the  plane  formed  by  the  two  vertical  axes.  Provide  a  detailed  design 
for  this  lever  if  it  is  to  be  produced  by  casting  and 

a.  When  only  100  pieces  are  required 

b.  When  1 0,000  pieces  are  required 


82  3      Casting  and  Foundry  Work 


4.  Design  a  micrometer  frame  for  each  of  the  following  cases: 

a.  The  gap  of  the  micrometer  is  1.0  inch  (25  mm),  and  the  distance  from  the  axis 
of  the  barrel  to  the  inner  side  of  the  frame  is  1 .5  inches  (37.5  mm).  The  maxi- 
mum load  on  the  anvil  is  22  lb  (100  N). 

b.  The  gap  of  the  micrometer  is  6  inches  (150  mm),  and  the  distance  from  the  axis 
of  the  barrel  to  the  inner  side  of  the  frame  is  4.0  inches  (100  mm).  The  maxi- 
mum load  on  the  anvil  is  22  lb  (100  N). 

Assume  that  production  volume  is  4000  pieces  and  that  one  of  the  various  casting 
processes  is  used. 

TIP:  Base  your  design  on  rigidity.  The  maximum  deflection  must  not  exceed 
0.1  of  the  smallest  reading  of  the  micrometer. 

5.  A  pulley  transmits  a  torque  of  600  lb  ft  (813.6  Nm)  to  a  shaft  that  is  \XA  inches 
(31  mm)  in  diameter.  The  outer  diameter  of  the  pulley  is  10  inches  (250  mm),  and 
it  is  to  be  driven  by  a  flat  belt  that  is  2  inches  (50  mm)  in  width.  Design  this  pul- 
ley if  it  is  to  be  manufactured  by  casting  and  500  pieces  are  required. 

6.  Design  a  hydraulic  jack  capable  of  lifting  1  ton  and  having  a  stroke  of  6  inches 
(150  mm).  The  jack  is  operated  by  a  manual  displacement  (plunger)  pump  that 
pumps  oil  from  a  reservoir  into  the  high-pressure  cylinder  through  two  spring- 
actuated  nonreturn  valves  to  push  the  ram  upward.  The  reservoir  and  the  high- 
pressure  cylinder  are  also  connected  by  a  conduit,  but  the  flow  of  oil  is  obstructed 
by  a  screw  that,  when  unscrewed,  relieves  the  pressure  of  the  cylinder  by  allow- 
ing high-pressure  oil  to  flow  back  into  the  reservoir  and  the  ram  then  to  be  pushed 
downward.  Provide  a  workshop  drawing  for  each  component,  as  well  as  an  as- 
sembly drawing  for  the  jack.  Steel  balls  and  springs  are  to  be  purchased.  Assume 
production  volume  is  5000  pieces. 

7.  Design  a  table  for  the  machine  shop.  That  table  should  be  4  feet  (1.2  m)  in  height, 
with  a  surface  area  of  3  by  3  feet  (0.90  by  0.9  m),  and  should  be  able  to  carry  a 
load  of  half  a  ton.  Assume  production  volume  is  2000  pieces. 

8.  Design  a  little  wrench  for  loosening  and  tightening  nuts  and  bolts  of  a  bicycle.  The 
nut  size  is  5/8  inch  (15  mm),  and  the  required  torque  is  about  1.0  lb  ft  (1.356 
Nm).  Assume  production  volume  is  10,000  pieces. 

9.  A  straight-toothed  spur-gear  wheel  transmits  1200  lb  ft  (1627  Nm)  of  torque  to  a 
steel  shaft  that  is  2  inches  (50  mm)  in  diameter.  The  pitch  diameter  of  the  gear  is 
8  inches  (200  mm),  its  width  is  3  inches  (75  mm),  and  the  base  diameter  is  7.5 
inches  (187.5  mm).  Design  this  gear's  blank.  Assume  production  volume  is  4000 
pieces. 

10.  Design  a  frame  for  an  open-arch  (C-type)  screw  press  that  can  deliver  a  load  of  up 
to  2  tons.  The  open  gap  is  2  feet  (600  mm),  and  the  bed  on  which  workpieces  are 
placed  is  12  by  12  inches  (300  by  300  mm).  Assume  that  the  base  diameter  of  the 
screw  thread  is  1  Vi  inches  (37.5  mm). 


Chapter  3  Design  Projects  83 

11.  Design  a  hydraulic  cylinder  for  earth-moving  equipment.  It  can  generate  a  maxi- 
mum force  of  2  tons  and  has  a  stroke  of  4  feet  (1200  mm).  Although  the  maximum 
force  is  generated  only  when  the  plunger  rod  is  moving  out,  the  cylinder  is  dou- 
ble acting  and  generates  a  force  of  1  ton  during  its  return  stroke.  Expected  pro- 
duction volume  is  2000  pieces,  and  the  pistons,  oil  rings,  and  so  on,  are  going  to 
be  purchased  from  vendors. 

12.  Design  a  safety  valve  to  be  mounted  on  a  high-pressure  steam  boiler.  The  pipe  on 
which  it  will  be  mounted  has  a  bore  diameter  of  2  inches  (50  mm).  The  pressure 
inside  the  boiler  should  not  exceed  50  folds  of  the  atmospheric  pressure.  Expected 
production  volume  is  5000  pieces,  and  the  stems,  springs,  bolts,  and  gaskets  are 
going  to  be  purchased  from  vendors. 


Chapter  4 


Inlng  of  Metals 


^s— "*^& 


IVETING 


INTRODUCTION 

When  two  parts  of  metal  are  to  be  attached  together,  the  resulting  joint  can  be 
made  dismountable  (using  screws  and  the  like),  or  it  can  be  made  permanent 
by  employing  riveting,  welding,  or  brazing  processes.  The  design  of  dismount- 
able joints  falls  beyond  the  scope  of  this  text  and  is  covered  in  machine  de- 
sign. It  is,  therefore,  the  aim  of  this  chapter  to  discuss  the  design  and 
production  of  permanent  joints  when  various  technologies  and  methods  are  ap- 
plied. Because  the  same  equipment  used  in  welding  is  also  sometimes  em- 
ployed in  the  cutting  of  plates,  thermal  cutting  processes  will  also  be 
discussed  in  this  chapter. 


The  process  of  riveting  involves  inserting  a  ductile  metal  pin  through  holes  in  two  or 
more  sheet  metals  and  then  forming  over  (heading)  the  ends  of  the  metal  pin  so  as  to 
secure  the  sheet  metals  firmly  together.  This  process  can  be  performed  either  cold  or 
hot,  and  each  rivet  is  usually  provided  with  one  preformed  head.  Figure  4.1a  and  b  in- 
dicates the  sequence  of  operations  in  riveting,  while  Figure  4.2  illustrates  different 
shapes  of  preformed  rivet  heads. 


ELDING 


Welding  is  the  joining  of  two  or  more  pieces  of  metal  by  creating  atom-to-atom  bonds 
between  the  adjacent  surfaces  through  the  application  of  heat,  pressure,  or  both.  In 
order  for  a  welding  technique  to  be  industrially  applicable,  it  must  be  reasonable  in 
cost,  yield  reproducible  or  consistent  weld  quality,  and,  more  importantly,  produce 


84 


4.2  Welding 


85 


FIGURE    4.1 

Sequence  of  operations 
in  riveting:  (a)  flat-head 
rivet;  (b)  regular  rivet 


Hammering 


vzz&  m%, 


w&.  ^m 


Lower 

tool 

— V— 


(a) 


(b) 


joints  with  properties  comparable  to  those  of  the  base  material.  Various  welding  tech- 
niques have  been  developed  that  are  aimed  at  achieving  these  three  goals.  However,  no 
matter  what  welding  method  is  used,  the  interface  between  the  original  two  parts  must 
disappear  if  a  strong  joint  is  to  be  obtained.  Before  we  discuss  the  different  methods 
employed  to  make  those  surfaces  disappear,  let  us  discuss  joint  design  and  preparation. 

Joint  Design  and  Preparation 

A  weld  joint  must  be  designed  to  withstand  the  forces  to  which  it  is  going  to  be  sub- 
jected during  its  service  life.  Therefore,  the  joint  design  is  determined  by  the  type  and 
magnitude  of  the  loading  that  is  expected  to  act  on  the  weldment.  In  other  words,  se- 
lection of  the  type  of  joint  has  to  be  made  primarily  on  the  basis  of  load  requirement. 
As  Figure  4.3a  through  e  shows,  there  are  five  types  of  weld  joints:  butt,  lap,  corner, 
T,  and  edge.  Following  is  a  discussion  of  each  of  these  different  types  of  joints. 

Butt  joint.  The  butt  joint  involves  welding  the  edges  or  end  faces  of  the  two  original 
parts,  as  shown  in  Figure  4.3a.  Therefore,  the  two  parts  must  be  aligned  in  the  same 
plane.  Usually,  when  the  thickness  of  the  parts  falls  between  1/8  and  3/8  inch  (about  3 
and  9  mm),  the  two  parts  are  welded  without  any  edge  preparation.  This  type  of  weld 
is  referred  to  as  a  square  weld  and  can  be  either  single  or  double,  depending  upon  the 
thickness  of  the  metal,  as  shown  in  Figure  4.4a.  As  can  be  seen  in  Figure  4.4b  through 
e,  the  edges  of  thicker  parts  should  be  prepared  with  single  or  double  bevels  or  V-,  J-, 
or  U-grooves  to  allow  adequate  access  to  the  root  of  the  joint.  Usually,  it  is  recom- 
mended to  adopt  the  single  or  double  U-groove  when  the  thickness  of  the  parts  is  more 
than  0.8  inch  (20  mm). 

Lap  joint.  We  can  see  in  Figure  4.3b  that  the  lap  joint  is  produced  by  fillet  welding 
overlapping  members;  the  amount  of  overlap  is  normally  taken  to  be  about  three  to 


FIGURE    4.2 

Different  shapes  of 
preformed  rivet  heads 


^Z7  c 


86 


4      Joining  of  Metals 


FIGURE    4.3 

Types  of  weld  joints: 
(a)  butt  joint;  (b)  lap 
joint;  (c)  corner  joint; 
(d)  T-joint;  (e)  edge  joint 


1 \ 


(b) 


(0 


CZlAZZ] 


(d) 


(e) 


five  times  the  thickness  of  the  member.  The  fillet  weld  can  be  continuous  and  may  also 
be  of  the  plug  or  slot  type,  as  shown  in  Figure  4.5. 

Corner  joint.  Figure  4.3c  illustrates  the  corner  joint,  which  can  be  welded  with  or 
without  edge  preparation  (see  Figure  4.4  for  the  various  possible  edge  preparations). 

T-joint.  The  T-joint  is  shown  in  Figure  4.3d.  T-joints  that  will  be  subjected  to  light 
static  loads  may  not  require  edge  preparation.  On  the  other  hand,  edge  preparations 
(again  see  Figure  4.4)  are  often  necessary  for  greater  metal  thicknesses  or  when  the 
joint  is  to  be  subjected  to  relatively  high,  alternating,  or  impulsive  loading. 

Edge  joint.  The  edge  joint  is  usually  used  when  welding  thin  sheets  of  metal  with  a 
thickness  of  up  to  1/8  inch  (3  mm).  Notice  in  Figure  4.3e  that  the  edges  of  the  mem- 
bers must  be  bent  before  the  welding  process  is  carried  out. 


FIGURE    4.4 

Different  edge 
preparation  for  butt 
welding:  (a)  square;  (b) 
bevel;  (c)  V-groove;  (d) 
J-groove;  (e)  U-groove 


(Single) 
square 


(Double) 
square 


Single  Single  Single  Single 


(a) 


Double 
(b) 


Double 
(c) 


Double 
(d) 


Double 
(e) 


FIGURE    4.5 

Basic  types  of  fusion 
lap  welds 


o  © 

£ 

3 

"K 


Fillet 


Plug 


Slot 


4.2  Welding 


87 


FIGURE    4.6 

Weld  symbols 


j^y 


m 


v. 


Groove  weld 
symbol 


rry 


n^ 


Fillet 
Groove  welds: 


V 


Square 


Plug  or  slot 


7V 


/ 


Back  or  backing  Spot  or  projection  Seam 


Vnv 


a 


/ 


Bevel 


h 


V 


Weld  Symbols  and  Identification 

Figure  4.6  shows  the  different  weld  symbols,  whereas  Figure  4.7  shows  the  standard 
identification  of  welds  employed  in  design  drawings. 

Classification  of  the  Welding  Processes 

Different  methods  can  be  used  for  classifying  industrial  welding  processes.  Each 
method  is  employed  to  form  groups  of  welding  processes,  with  each  group  having 
something  in  common.  For  instance,  welding  processes  can  be  classified  according  to 
the  source  of  energy  required  to  accomplish  welding.  In  such  a  case,  it  is  obvious  that 
there  are  four  main  groups:  mechanical,  electrical,  chemical,  or  optical.  Welding 
processes  can  also  be  classified  by  the  degree  of  automation  adopted,  which  yields 
three  groups:  manual,  semiautomatic,  and  automatic.  The  most  commonly  used 
method  of  classification  is  according  to  the  state  of  the  metal  at  the  locations  being 


FIGURE    4.7 

Standard  identification 
of  welds 


Basic  weld  symbol 
or  detail  reference 

Size;  size  or  strength 
for  resistance  welds 


Reference  line 


Root  opening;  depth 

of  filling  for  plug 

and  slot  welds 


Specification,  process, 
or  other  reference 


Tail  (may  be  omitted 

when  reference 

is  not  used) 


Basic  weld  symbol 
or  detail  reference 


Finish 
symbol 

Contour 
symbol 

Groove  angle  included 
angle  or  countersink 
for  plug  weld 
Pitch  (center-to-center 
spacing)  of  welds 
Arrow  connecting  reference 
line  to  arrow  side  of  joint, 
to  grooved  member,  or  both 


(N) 

Number  of  spots 
or  projection  welds 

Elements  in  this  area 

remain  as  shown 

when  tail  and  arrow 

are  reversed 


y 


Field  weld 
symbol 


Weld 

all  around 

symbol 


88  4     Joining  of  Metals 


welded,  thus  splitting  the  welding  processes  into  two  main  categories:  pressure  weld- 
ing and  fusion  welding.  We  now  discuss  each  of  these  two  categories  in  detail. 

Pressure  Welding  Processes 

Pressure  welding  involves  processes  in  which  the  application  of  external  pressure  is 
indispensable  to  the  production  of  weld  joints  formed  either  at  temperatures  below  the 
melting  point  (solid-state  welding)  or  at  temperatures  above  the  melting  point  (fusion 
welding).  In  both  cases,  it  is  important  to  have  very  close  contact  between  the  atoms 
of  the  parts  that  are  to  be  joined.  The  atoms  must  be  moved  together  to  a  distance  that 
is  equal  to  or  less  than  the  equilibrium  interatomic-separation  distance.  Unfortunately, 
there  are  two  obstacles  that  must  be  overcome  so  that  successful  pressure  welding  can 
be  carried  out  and  a  sound  weldment  can  be  obtained.  First,  surfaces  are  not  flat  when 
viewed  on  a  microscopic  scale.  Consequently,  intimate  contact  can  be  achieved  only 
where  peaks  meet  peaks,  as  can  be  seen  in  Figure  4.8,  and  the  number  of  bonds  would 
not  be  enough  to  produce  a  strong  welded  joint.  Second,  the  surfaces  of  metals  are  usu- 
ally covered  with  oxide  films  that  inhibit  direct  contact  between  the  two  metal  parts  to 
be  welded.  Therefore,  those  oxide  and  nonmetallic  films  must  be  removed  (cleaned 
with  a  wire  brush)  before  welding  in  order  to  ensure  a  strong  welded  joint.  Pressure 
welding  processes  are  applied  primarily  to  metals  possessing  high  ductility  or  those 
whose  ductility  increases  with  increasing  temperatures;  thus,  the  peaks  that  keep  the 
surfaces  of  the  two  metallic  members  apart  are  leveled  out  under  the  action  of  me- 
chanical stresses  or  the  combined  effect  of  high  temperatures  and  mechanical  stresses. 
In  fact,  a  wide  variety  of  pressure  welding  processes  are  used  in  industry.  The  com- 
monly used  ones  are  listed  in  Figure  4.9. 

Cold-pressure  welding.  Cold-pressure  welding  is  a  kind  of  solid-state  welding  used 
for  joining  sheets,  wires,  and  small  electric  components.  As  previously  discussed,  the 
surfaces  to  be  welded  must  be  cleaned  with  a  wire  brush  to  remove  the  oxide  film 
and  must  be  carefully  degreased  before  welding.  As  Figure  4.10  shows,  a  special 
tool  is  used  to  produce  localized  plastic  deformation,  which  results  in  coalescence  be- 
tween the  two  parts.  This  process,  which  can  replace  riveting,  is  usually  followed  by 


FIGURE    4.8 

A  microscopic  view  of 

two  mating  surfaces  Surface  1 


Surface  2 


4.2  Welding 


89 


FIGURE    4.9 

Classification  of  the 
commonly  used 
pressure  welding 
processes 


Pressure  welding  processes 


Cold-pressure  welding 

*  Cold-pressure  welding 
of  sheets  and  wires 

*  Ultrasonic  welding 

*  Explosive  welding 


Hot-pressure  welding 


Molten-metal  bonding 

*  Percussion 

*  Resistance  flash 

*  Resistance  spot 

*  Resistance  seam 

*  Resistance  projection 

*  Thermit 


Hot  solid-state 
pressure  welding 

*  Diffusion  bonding 

*  Friction  welding 

*  Inertia  welding 

*  Induction  welding 
«  Resistance  upset 

(butt)  welding 


annealing  of  the  welded  joint.  Figure  4.10  also  shows  that  recrystallization  takes  place 
during  the  annealing  operation.  This  is  added  to  diffusion,  which  finally  results  in  com- 
plete disappearance  of  the  interface  between  the  two  parts. 

Cold-pressure  welding  of  wires  is  performed  by  means  of  a  special-purpose  ma- 
chine. Figure  4. 1 1  illustrates  the  steps  involved  in  this  process.  As  can  be  seen,  the 
wires'  ends  are  clamped  and  pressed  repeatedly  against  each  other  in  order  to  ensure 
adequate  plastic  deformation.  The  excess  upset  metal  is  then  trimmed  by  the  sharp 
edges  of  the  gripping  jaws.  This  technique  is  used  when  welding  wires  of  nonferrous 
metals  such  as  aluminum,  copper,  or  aluminum-copper  alloys. 

Explosive  welding.  Explosive  welding  is  another  technique  that  produces  solid-state 
joints  and,  therefore,  eliminates  the  problems  associated  with  fusion  welding  methods, 
like  the  heat-affected  zone  and  the  microstructural  changes.  The  process  is  based  on 
using  high  explosives  to  generate  extremely  high  pressures  that  are,  in  turn,  used  to 
combine  flat  plates  or  cylindrical  shapes  metallurgically.  Joints  of  dissimilar  metals 
and/or  those  that  are  extremely  difficult  to  combine  using  conventional  methods  can 
easily  be  produced  by  explosive  welding. 

During  explosive  welding,  a  jet  of  soft  (or  fluidlike)  metal  is  formed  (on  a  micro- 
scopic scale)  and  breaks  the  oxide  film  barrier  to  bring  the  two  metal  parts  into  inti- 
mate contact.  That  metal  jet  is  also  responsible  for  the  typical  wavy  interface  between 


FIGURE    4.10 

Cold-pressure  welding 
of  sheets 


Welds 


T? 


W 


Punch 


Recrystallization 


r?i 


Punch 


Welded  joint  after 
deformation 


Welded  joint  after 
annealing 


90 


4      Joining  of  Metals 


FIGURE    4.11 

Cold-pressure  welding 
of  wires 


Clamps 


Upset  metal 


J2, 


S     £ 


After  trimming 


the  two  metal  parts,  thus  creating  mechanical  interlocking  between  them  and,  finally, 
resulting  in  a  strong  bond.  Figure  4. 1 2  illustrates  an  arrangement  for  explosive  welding 
two  flat  plates,  and  Figure  4.13  is  a  magnified  sketch  of  the  wavy  interface  between 
explosively  welded  parts. 


FIGURE    4.12 

An  arrangement  for 
explosive  welding  two 
flat  plates 


Detonator 


Explosive 
layer 


L 


^Ml^M^ 


Plate  1 


Plate  2 


plate 


Explosive  welding  and  explosive  cladding  are  popular  in  the  manufacture  of  heat 
exchangers  and  chemical-processing  equipment.  Armored  and  reinforced  composites 
with  a  metal  matrix  are  also  produced  by  explosive  welding.  Nevertheless,  a  clear 
limitation  is  that  the  process  cannot  be  used  successfully  for  welding  hard,  brittle  met- 
als. Research  is  being  carried  out  in  this  area,  and  new  applications  are  continuously 
introduced. 

Ultrasonic  welding.  The  ultrasonic  welding  method  of  solid-state  welding  is  com- 
monly used  for  joining  thin  sheets  or  wires  of  similar  or  dissimilar  metals  in  order 
to  obtain  lap-type  joints.  Mechanical  vibratory  energy  with  ultrasonic  frequencies  is 
applied  along  the  interfacial  plane  of  the  joint,  while  a  nominal  static  stress  is  applied, 
normal  to  that  interface,  to  clamp  the  two  components  together.  Oscillating  shear 


FIGURE    4.13 

A  sketch  of  the  wavy 
interface  between 
explosively  welded 
parts 


Interface 


4.2  Welding  91 

stresses  are,  therefore,  initiated  at  the  interface  and  disperse  surface  films,  allowing  in- 
timate contact  between  the  two  metals,  and,  consequently,  producing  a  strong  joint.  Ul- 
trasonic welding  does  not  involve  the  application  of  high  pressures  or  temperatures 
and  is  accomplished  within  a  short  time.  Therefore,  this  process  is  especially  suitable 
for  automation  and  has  found  widespread  application  in  the  electrical  and  microelec- 
tronics industries  in  the  welding  of  thin  metal  foils  for  packaging  and  splicing  and  in 
the  joining  of  dissimilar  materials  in  the  fabrication  of  nuclear  reactor  components.  It 
must  be  noted,  however,  that  the  process  is  restricted  to  joining  thin  sheets  or  fine 
wires.  Nevertheless,  this  restriction  applies  only  to  thinner  pieces,  and  the  process  is 
often  used  in  welding  thin  foils  to  thicker  sheets. 

Different  types  of  ultrasonic  welding  machines  are  available,  each  constructed  to 
produce  a  certain  type  of  weld,  such  as  spot,  line,  continuous  seam,  or  ring.  A  sketch 
of  a  spot-type  welding  machine  that  is  commonly  used  in  welding  microcircuit  ele- 
ments is  illustrated  in  Figure  4.14.  As  we  can  see,  the  machine  consists  basically  of  a 
frequency  convenor  that  transforms  the  standard  60-Hz  (or  50-Hz  in  Europe)  electric 
current  into  a  high-frequency  current  (with  a  fixed  frequency  in  the  range  of  15  to  75 
kHz),  a  transducer  that  converts  the  electrical  power  into  elastic  mechanical  vibrations, 
and  a  horn  that  magnifies  the  amplitude  of  these  vibrations  and  delivers  them  to  the 
weld  zone.  Other  associated  elements  include  the  anvil,  a  force-application  and  clamp- 
ing device,  a  sonotrode  (as  compared  with  the  electrode  in  resistance  welding),  and  ap- 
propriate controls  to  set  up  optimal  values  for  the  process  variables,  such  as  vibratory 
power  and  weld  time. 

Friction  welding.  In  friction  welding,  a  type  of  hot  solid-state  welding,  the  parts  to  be 
welded  are  tightly  clamped,  one  in  a  stationary  chuck  and  the  other  in  a  rotatable 
chuck  that  is  mounted  on  a  spindle.  External  power  is  employed  to  drive  the  spindle  at 
a  constant  speed,  with  the  two  parts  in  contact  under  slight  pressure.  Kinetic  energy  is 
converted  to  frictional  heat  at  the  interface.  When  the  mating  edges  of  the  workpieces 
attain  a  suitable  temperature  (in  the  forging  range)  that  permits  easy  plastic  flow,  the 
spindle  rotation  is  halted,  and  high  axial  pressure  is  applied  to  plastically  deform  the 
metal,  obtain  intimate  contact,  and  produce  a  strong,  solid  weld.  This  is  clearly  shown 
in  Figure  4.15,  which  indicates  the  stages  involved  in  friction  welding. 

Several  advantages  have  been  claimed  for  the  friction  welding  process.  These  in- 
clude simplicity,  high  efficiency  of  energy  utilization,  and  the  ability  to  join  similar  as 


FIGURE    4.14  Clamping 

A  sketch  of  an  Transducer  H°m         *?* 

ultrasonic  spot-type 
welding  machine 


Workpieces 


92  4      Joining  of  Metals 


FIGURE    4.15 

Stages  involved  in 
friction  welding 


Low 
thrust 


IBEE3  -EEEEE3-  E--EE3- 


OAT 

Rotating  Heating  stage  Upsetting  stage 

well  as  dissimilar  metal  combinations  that  cannot  be  joined  by  conventional  welding 
methods  (e.g.,  aluminum  to  steel  or  aluminum  to  copper).  Also,  since  contaminants 
and  oxide  films  are  carried  away  from  the  weld  area  where  grain  refinement  takes 
place,  a  sound  bond  is  obtained  and  usually  has  the  same  strength  as  the  base  metal. 
Nevertheless,  a  major  limitation  of  the  process  is  that  at  least  one  of  the  two  parts  to 
be  joined  must  be  a  body  of  revolution  around  the  axis  of  rotation  (like  a  round  bar  or 
tube).  A  further  limitation  is  that  only  forgeable  metals  that  do  not  suffer  from  hot 
shortness  can  successfully  be  friction  welded.  Also,  care  must  be  taken  during  welding 
to  ensure  squareness  of  the  edges  of  workpieces  as  well  as  concentricity  of  round  bars 
or  tubes. 

Inertia  welding.  Inertia  welding  is  a  version  of  friction  welding  that  is  recom- 
mended for  larger  workpieces  or  where  high-strength  alloys  (i.e.,  superalloys)  are  to 
be  joined  together.  Inertia  welding,  as  the  name  suggests,  efficiently  utilizes  the  ki- 
netic energy  stored  in  a  rotating  flywheel  as  a  source  for  heating  and  for  much  of  the 
forging  of  the  weld.  As  is  the  case  with  friction  welding,  the  two  workpieces  to 
be  inertia  welded  are  clamped  tightly  in  stationary  and  rotatable  chucks,  the  differ- 
ence being  that  the  rotatable  chuck  is  rigidly  coupled  to  a  flywheel  in  the  case  of 
inertia  welding.  The  process  involves  rotating  the  flywheel  at  a  predetermined  angu- 
lar velocity  and  then  converting  the  kinetic  energy  of  the  freely  rotating  flywheel  to 
frictional  heat  at  the  weld  interface  by  applying  an  axial  load  to  join  the  abutting 
ends  under  controlled  pressure.  The  process  requires  shorter  welding  time  than  that 
taken  in  conventional  friction  welding,  especially  for  larger  workpieces.  Examples  of 
inertia-welded  components  include  hydraulic  piston  rods  for  agricultural  machinery, 
carbon  steel  shafts  welded  to  superalloy  turbocharger  wheels,  and  bar  stock  welded 
to  small  forgings. 

Induction  welding.  As  the  name  suggests,  induction  welding  is  based  on  the  phe- 
nomenon of  induction.  We  know  from  physics  (electricity  and  magnetism)  that  when 
an  electric  current  flows  in  an  inductor  coil,  another  electric  current  is  induced  in  any 
conductor  that  intersects  with  the  magnetic  flux.  In  induction  welding,  the  source  of 
heat  is  the  resistance,  at  the  abutting  workpieces'  interface,  to  the  flow  of  current  in- 
duced in  the  workpieces  through  an  external  induction  coil.  Figure  4.16  illustrates  the 
principles  of  induction  welding.  For  efficient  conversion  of  electrical  energy  into  heat 
energy,  high-frequency  current  is  employed,  and  the  process  is  usually  referred  to  as 
high-frequency  induction  welding  (HFIW).  Frequencies  in  the  range  of  300  to  450  kHz 
are  commonly  used  in  industry,  although  frequencies  as  low  as  10  kHz  are  also  in  use. 
It  is  always  important  to  remember  the  "skin  effect"  when  designing  an  induction- 
welded  joint.  This  effect  refers  to  the  fact  that  the  electric  current  flows  superficially 
(i.e.,  near  the  surface).  In  fact,  the  depth  of  the  layer  through  which  the  current  flows 
is  dependent  mainly  upon  the  frequency  and  the  electromagnetic  properties  of  the 


4.2  Welding 


93 


FIGURE    4.16 

Principles  of  induction 
welding 


Workpiece- 


^fiflflQflflV, 


r^ 


Coil        ^I- 
Flux 


Workpiece 


workpiece  metal.  Industrial  applications  of  induction  welding  include  butt  welding  of 
pipes  and  continuous-seam  welding  for  the  manufacture  of  seamed  pipes. 

Thermit  welding.  Thermit  welding  makes  use  of  an  exothermic  chemical  reaction  to 
supply  heat  energy.  That  reaction  involves  the  burning  of  thermit,  which  is  a  mixture 
of  fine  aluminum  powder  and  iron  oxide  in  the  form  of  rolling-mill  scale,  mixed  at  a 
ratio  of  about  1  to  3  by  weight.  Although  a  temperature  of  5400°F  (3000°C)  may  be 
attained  as  a  result  of  the  reaction,  localized  heating  of  the  thermit  mixture  up  to  at 
least  2400°F  (1300°C)  is  essential  in  order  to  start  the  reaction,  which  can  be  given  by 
the  following  chemical  formula: 

8A1  +  3Fe304  ->  9Fe  +  4A1203  +  heat  (4.1) 

As  we  can  see  from  the  formula,  the  outcome  is  very  pure  molten  iron  and  slag.  In  fact, 
other  oxides  are  also  used  to  produce  pure  molten  metals;  these  include  chromium, 
manganese,  or  vanadium,  depending  upon  the  parent  metals  to  be  welded. 

Usually,  the  thermit  welding  process  requires  the  application  of  pressure  in  order 
to  achieve  proper  coalescence  between  the  parts  to  be  joined.  However,  fusion  thermit 
welding  is  also  used;  it  does  not  require  the  application  of  force.  In  this  case,  the  re- 
sulting molten  metal  is  a  metallurgical  joining  agent  and  not  just  a  means  for  heating 
the  weld  area. 

Thermit  welding  is  used  in  joining  railroad  rails,  pipes,  and  thick  steel  sections,  as 
well  as  in  repairing  heavy  castings.  The  procedure  involves  fitting  a  split-type  refrac- 
tory mold  around  the  abutting  surfaces  to  be  welded,  igniting  the  thermit  mixture  using 
a  primer  (ignition  powder)  in  a  special  crucible,  and,  finally,  pouring  the  molten  metal 
(obtained  as  a  result  of  the  reaction)  into  the  mold.  Because  the  temperature  of  the 
molten  metal  is  about  twice  the  melting  point  of  steel,  the  heat  input  is  enough  to  fuse 
the  abutting  surfaces,  which  are  usually  pressed  together  to  give  a  sound  weld. 

Diffusion  bonding.  Diffusion  bonding  is  a  solid-state  welding  method  in  which  the 
surfaces  to  be  welded  are  cleaned  and  then  maintained  at  elevated  temperatures  under 
appropriate  pressure  for  a  long  period  of  time.  No  fusion  occurs,  deformation  is  lim- 
ited, and  bonding  takes  place  principally  due  to  diffusion.  As  we  know  from  metal- 
lurgy, the  process  parameters  are  pressure,  temperature,  and  time,  and  they  should  be 
adjusted  to  achieve  the  desired  results. 

Butt  welding.  Butt  welding  belongs  to  the  resistance  welding  group,  which  also  con- 
sists of  the  spot,  seam,  projection,  percussion,  and  flash  welding  processes.  All  of  these 


94 


4      Joining  of  Metals 


FIGURE    4.17 

Upset-butt  welding 


Clamping  dies 


Movable 


Workpieces 


Ac  power  supply 

operate  on  the  same  principle,  which  involves  heating  the  workpieces  as  a  result  of 
being  a  part  of  a  high-amperage  electric  circuit  and  then  applying  external  pressure  to 
accomplish  strong  bonding.  Consequently,  all  the  resistance  welding  processes  belong 
to  the  larger,  more  general  group  of  pressure  welding;  without  the  application  of  ex- 
ternal pressure,  the  weld  joint  cannot  be  produced. 

In  butt  welding,  sometimes  called  upset-butt  welding  or  just  upset  welding,  the 
parts  are  clamped  and  brought  in  solid  contact,  and  low-voltage  (1  to  3  V)  alternating 
current  is  switched  on  through  the  contact  area,  as  illustrated  in  Figure  4.17.  As  a  re- 
sult of  the  heat  generated,  the  metal  in  the  weld  zone  assumes  a  plastic  state  (above  the 
solidus)  and  is  gradually  squeezed  and  expelled  from  the  contact  area.  When  enough 
upset  metal  becomes  evident,  the  current  is  switched  off  and  the  welded  parts  are  re- 
leased. Figure  4.18  indicates  a  typical  upset  welding  cycle.  Note  that  upset  welding 
would  not  be  successful  for  larger  sections  because  these  cannot  be  uniformly  heated 
and  require  extremely  high-amperage  current.  Therefore,  the  process  is  limited  to 
welding  wires  and  rods  up  to  3/8  inch  (about  10  mm)  in  diameter.  Also,  a  sound  joint 
can  be  ensured  only  when  the  two  surfaces  being  welded  together  have  the  same  cross- 
sectional  area  as  well  as  negligible  or  no  eccentricity. 

Flash  welding.  Flash  welding  is  somewhat  similar  to  upset  welding.  The  equipment 
for  flash  welding  includes  a  low-voltage  transformer  (5  to  10  V),  a  current  timing 


FIGURE    4.18 

A  typical  upset  welding 
cycle 


Pressure 
force 


Solid  contact 


Upset  metal 


4.2  Welding 


95 


FIGURE    4.19 

Stages  in  a  flash 
welding  cycle 


2    £ 


Localized 
bridges 


1      I 


Flashing 


Upset 
metal 


JjL 


Flashing  Upsetting 


Time 


device,  and  a  mechanism  to  compress  the  two  workpieces  against  each  other.  Figure 
4.19  illustrates  the  different  stages  involved  in  a  flash  welding  cycle.  We  can  see  that 
the  pressure  applied  at  the  beginning  is  low.  Therefore,  there  are  a  limited  number  of 
contact  points  that  act  as  localized  bridges  for  the  electric  current.  Consequently,  metal 
is  heated  at  those  points  when  the  current  is  switched  on,  and  the  temperature  increases 
with  the  increasing  current  until  it  exceeds  the  melting  point  of  the  metal.  At  this  stage, 
the  molten  metal  is  expelled  from  the  weld  zone,  causing  "flashing."  New  bridges  are 
formed  and  move  quickly  across  the  whole  interface,  resulting  in  uniform  heating  all 
over.  When  the  whole  contact  area  is  heated  above  the  liquidus  line,  electric  current  is 
switched  off,  and  the  pressure  is  suddenly  increased  to  squeeze  out  the  molten  metal, 
upset  the  abutted  parts,  and  weld  them  together. 

Flash  welding  is  used  for  joining  large  sections,  rails,  chain  links,  tools,  thin- 
walled  tubes,  and  the  like.  It  can  also  be  employed  for  welding  dissimilar  metals.  The 
advantages  claimed  for  the  process  include  its  higher  productivity  and  its  ability  to 
produce  high-quality  welds.  The  only  disadvantage  is  the  loss  of  some  metal  in 
flashing. 

Percussion  welding.  In  percussion  welding,  a  method  of  resistance  welding,  a  high- 
intensity  electric  current  is  discharged  between  the  parts  before  they  are  brought  in 
solid  contact.  This  results  in  an  electric  arc  in  the  gap  between  the  two  surfaces.  That 
electric  arc  lasts  only  for  about  0.001  second  and  is  enough  to  melt  the  surfaces  to  a 
depth  of  a  few  thousandths  of  an  inch.  The  two  parts  are  then  impacted  against  each 
other  at  a  high  speed  to  obtain  a  sound  joint.  The  major  limitation  of  this  process  is 
the  cross-sectional  area  of  the  welded  joint.  It  should  not  exceed  0.5  square  inch 
(300  mm2)  in  order  to  keep  the  intensity  of  current  required  at  a  practical  level.  In  in- 
dustry, percussion  welding  is  limited  to  joining  dissimilar  metals  that  cannot  be  welded 
otherwise. 

Spot  welding.  Figure  4.20a  illustrates  the  principles  of  operation  of  spot  welding,  a  re- 
sistance welding  process.  Electric  current  is  switched  on  between  the  welding  electrodes 


96 


4     Joining  of  Metals 


FIGURE    4.20 

Resistance  spot 
welding:  (a)  principles 
of  operation;  (b)  a 
cross  section  through  a 
spot  weld 


Copper 
electrode 


CXJ 


Ac  power 
supply 


(a) 


Dendrites 
(featherlike- 
structure) 


Force 


Time 


-X, 


Nugget 


(b) 


to  flow  through  the  lapped  sheets  (workpieces)  that  are  held  together  under  pressure.  As 
can  be  seen  in  Figure  4.20b,  the  metal  fuses  in  the  central  area  of  the  interface  between 
the  two  sheets  and  then  solidifies  in  the  form  of  a  nugget,  thus  providing  the  weld  joint. 
Heat  is  also  generated  at  the  contact  areas  between  the  electrodes  and  the  workpieces. 
Therefore,  some  precautions  must  be  taken  to  prevent  excessive  temperatures  and  fus- 
ing of  the  metal  at  those  spots.  The  electrodes  used  must  possess  good  electrical  and 
thermal  conductivities.  They  are  usually  hollow  and  are  water-cooled.  In  addition,  areas 
of  workpieces  in  contact  with  the  electrodes  must  be  cleaned  carefully. 

Spot  welding  is  the  most  widely  used  resistance  welding  process  in  industry.  Car- 
bon steel  sheets  having  a  thickness  up  to  0. 1 25  inch  (4  mm)  can  be  successfully  spot 
welded.  Spot-welding  machines  have  ratings  up  to  more  than  600  kVA  and  use  a  volt- 
age of  1  to  12  V  obtained  from  a  step-down  transformer.  Multispot  machines  are  used, 
and  the  process  can  be  fully  automated.  Therefore,  spot  welding  has  found  widespread 
application  in  the  automobile,  aircraft,  and  electronics  industries,  as  well  as  in  sheet 
metal  work. 

Seam  welding.  Seam  welding  and  projection  welding  are  modifications  of  spot  weld- 
ing. In  seam  welding,  the  lapped  sheets  are  passed  between  rotating  circular  electrodes 
through  which  the  high-amperage  current  flows,  as  shown  in  Figure  4.21.  Electrodes 
vary  in  diameter  from  less  than  2  up  to  14  inches  (40  to  350  mm),  depending  upon  the 
curvature  of  the  workpieces  being  welded.  Welding  current  as  high  as  5000  A  may  be 
employed,  and  the  pressing  force  acting  upon  the  electrodes  can  go  up  to  6  kN  (more 
than  half  a  ton).  A  welding  speed  of  about  12  feet  per  minute  (4  m/min.)  is  quite  com- 
mon. Seam  welding  is  employed  in  the  production  of  pressure-tight  joints  used  in  con- 
tainers, tubes,  mufflers,  and  the  like.  Advantages  of  this  process  include  low  cost,  high 


4.2  Welding 


97 


FIGURE    4.21 

Principles  of  seam 
welding 


Forces 


Forces 


Sheet  metal 


Sheet  metal 


Overlapping  nuggets 


production  rates,  and  suitability  for  automation.  Nevertheless,  the  thickness  of  the 
sheets  to  be  seam  welded  is  limited  to  0.125  inch  (4  mm)  in  the  case  of  carbon  steels 
and  much  less  for  more  conductive  alloys  due  to  the  extremely  high  amperage  required 
(0.125-inch-thick  steel  sheets  require  19,000  A,  whereas  aluminum  sheets  having  the 
same  thickness  would  require  76,000  A). 

Projection  welding.  In  projection  welding,  one  of  the  workpieces  is  purposely  pro- 
vided with  small  projections  so  that  current  flow  and  heating  are  localized  at  those 
spots.  The  projections  are  usually  produced  by  die  pressing,  and  the  process  calls  for 
the  use  of  a  special  upper  electrode.  Figure  4.22  illustrates  an  arrangement  of  two  parts 
to  be  projection  welded,  as  well  as  the  resulting  weld  nugget.  As  you  may  expect,  the 
projections  collapse  under  the  externally  applied  force  after  sufficient  heating,  thus 
yielding  a  well-defined,  fused  weld  nugget.  When  the  current  is  switched  off,  the  weld 
cools  down  and  solidification  takes  place  under  the  applied  force.  The  electrode  force 
is  then  released,  and  the  welded  workpiece  is  removed.  As  is  the  case  with  spot  weld- 
ing, the  entire  projection  welding  process  takes  only  a  fraction  of  a  second. 

Projection  welding  has  some  advantages  over  conventional  spot  welding.  For  in- 
stance, sheets  that  are  too  thick  to  be  joined  by  spot  welding  can  be  welded  using  this 
process.  Also,  the  presence  of  grease,  dirt,  or  oxide  films  on  the  surface  of  the  work- 
pieces  has  less  effect  on  the  weld  quality  than  in  the  case  of  spot  welding.  A  further 


FIGURE    4.22 

An  arrangement  for 
projection  welding  two 
parts 


Pressure 


I 7 


k\\\\\\\\\\\\\\\\\\\^^,,,„..r 


* 


4zzzzJ 


ffir 


7\ 


After  welding 


Pressure 


98 


4      Joining  of  Metals 


advantage  of  projection  welding  is  the  accuracy  of  locating  welds  inherent  in  that 
process. 

Fusion  Welding  Processes 

Fusion  welding  includes  a  group  of  processes  that  all  produce  welded  joints  as  a  result 
of  localized  heating  of  the  edges  of  the  base  metal  above  its  melting  temperature, 
wherein  coalescence  is  produced.  A  filler  metal  may  or  may  not  be  added,  and  no  ex- 
ternal pressure  is  required.  The  welded  joint  is  obtained  after  solidification  of  the  fused 
weld  pool. 

In  order  to  join  two  different  metals  together  by  fusion  welding,  they  must  possess 
some  degree  of  mutual  solubility  in  the  solid  state.  In  fact,  metals  that  are  completely 
soluble  in  the  solid  state  exhibit  the  highest  degree  of  weldability.  Metals  with  limited 
solid  solubility  have  lower  weldability,  and  metals  that  are  mutually  insoluble  in  the 
solid  state  are  completely  unweldable  by  any  of  the  fusion  welding  methods.  In  that 
case,  an  appropriate  pressure  welding  technique  should  be  employed.  An  alternative 
solution  is  to  employ  an  intermediate  metal  that  is  soluble  in  both  base  metals. 

Metallurgy  of  fusion  welding.  Before  surveying  the  different  fusion  welding 
processes,  let  us  discuss  the  metallurgy  of  fusion  welding.  Important  microstructural 
changes  take  place  in  and  around  the  weld  zone  during  and  after  the  welding  operation. 
Such  changes  in  the  microstructure  determine  the  mechanical  properties  of  the  welded 
joint.  Therefore,  a  study  of  the  metallurgy  of  fusion  welding  is  essential  for  good  de- 
sign of  welded  joints,  as  well  as  for  the  optimization  of  the  process  parameters. 

During  fusion  welding,  three  zones  can  be  identified,  as  shown  in  Figure  4.23, 
which  indicates  a  single  V-weld  in  steel  after  solidification  and  the  corresponding  tem- 
perature distribution  during  welding.  In  the  first  zone,  called  the  fusion  zone,  the  base 
metal  and  deposited  metal  (if  a  filler  rod  is  used)  are  brought  to  the  molten  state  dur- 
ing welding.  Therefore,  when  this  zone  solidifies  after  welding,  it  generally  has  a 
columnar  dendritic  structure  with  haphazardly  oriented  grains.  In  other  words,  the  mi- 
crostructure of  this  zone  is  quite  similar  to  that  of  the  cast  metal.  Nevertheless,  if  the 
molten  metal  is  overheated  during  welding,  this  results  in  an  acicular  structure  that  is 
brittle,  has  low  strength,  and  is  referred  to  as  the  Widmanstatten  structure.   Also,  the 


FIGURE    4.23 

The  three  zones  in  a 
fusion-welded  joint  and 
the  temperature 
distribution  during 
welding 


1500°C 


700°C 


2700° F 


1300°F 


Fusion  zone 


Parent  metal 


Heat-affected 
zones 


4.2  Welding  99 

chemical  composition  of  the  fusion  zone  may  change,  depending  upon  the  kind  and 
amount  of  filler  metal  added. 

The  second  zone,  which  is  referred  to  as  the  heat-affected  zone  (HAZ),  is  that  por- 
tion of  the  base  metal  that  has  not  been  melted.  Therefore,  its  chemical  composition 
before  and  after  welding  remains  unchanged.  Nevertheless,  its  microstructure  is  al- 
ways altered  because  of  the  rapid  heating  during  welding  and  subsequent  cooling.  In 
fact,  the  HAZ  is  subjected  to  a  normalizing  operation  during  welding  and  may  conse- 
quently undergo  phase  transformations  and  precipitation  reactions,  depending  upon  the 
nature  (chemical  composition  and  microstructure)  of  the  base  metal.  The  size  of  the 
HAZ  is  dependent  upon  the  welding  method  employed  and  the  nature  of  the  base 
metal.  This  can  be  exemplified  by  the  fact  that  the  HAZ  is  0.1  inch  (2.5  mm)  when  au- 
tomatic submerged  arc  welding  is  used,  ranges  from  0.2  to  0.4  inch  (5  to  10  mm)  for 
shielded-metal  arc  welding,  and  may  reach  1  inch  (25  mm)  in  conventional  gas  weld- 
ing. This  evidently  affects  the  microstructure  of  the  weld,  which  is  generally  fine- 
grained. The  effect  of  these  structural  changes  on  the  mechanical  properties  of  the 
weld  differs  for  different  base  metals.  For  instance,  the  structural  changes  have  negli- 
gible effect  on  the  mechanical  properties  of  low-carbon  steel,  regardless  of  the  weld- 
ing method  used.  On  the  contrary,  when  welding  high-carbon  alloy  steel,  hardened 
structures  like  maternsite  are  formed  in  the  HAZ  of  the  weld  that  result  in  a  sharp  re- 
duction in  the  ductility  of  the  welded  joint  and/or  crack  formation.  (Remember  the  ef- 
fect of  alloying  elements  on  the  critical  cooling  rate  in  the  TTT  diagram  that  you 
studied  in  metallurgy.) 

The  third  zone  involves  the  unaffected  parent  metal  adjacent  to  the  HAZ  that  is 
subjected  to  a  temperature  below  AC3  (a  critical  temperature)  during  welding.  In  this 
zone,  no  structural  changes  take  place  unless  the  base  metal  has  been  subjected  to  plas- 
tic deformation  prior  to  welding,  in  which  case  recrystallization  and  grain  growth 
would  become  evident. 

Arc  welding.  Arc  welding  is  based  on  the  thermal  effect  of  an  electric  arc  that  is  act- 
ing as  a  powerful  heat  source  to  produce  localized  melting  of  the  base  metal.  The  elec- 
tric arc  is,  in  fact,  a  sustained  electrical  discharge  (of  electrons  and  ions  in  opposite 
directions)  through  an  ionized,  gaseous  path  between  two  electrodes  (i.e.,  the  anode 
and  the  cathode).  In  order  to  ionize  the  air  in  the  gap  between  the  electrodes  so  that  the 
electric  arc  can  consequently  be  started,  a  certain  voltage  is  required.  (The  voltage  re- 
quired depends  upon  the  distance  between  the  electrodes.)  The  ionization  process  re- 
sults in  the  generation  of  electrons  and  positively  charged  ions.  Next,  the  electrons 
impact  on  the  anode,  and  the  positively  charged  ions  impact  on  the  cathode.  The  col- 
lisions of  these  particles,  which  are  accelerated  by  the  arc  voltage,  transform  the  ki- 
netic energy  of  the  particles  into  thermal  and  luminous  energy,  and  the  temperature  at 
the  center  of  the  arc  can  reach  as  high  as  11,000°F  (6000°C).  Actually,  only  a  com- 
paratively low  potential  difference  between  the  electrodes  is  required  to  start  the  arc. 
For  instance,  about  45  V  is  usually  sufficient  for  direct  current  (dc)  welding  equipment, 
and  up  to  60  V  for  an  alternating  current  (ac)  welder.  Also,  the  voltage  drops  after  the 
arc  is  started,  and  a  stable  arc  can  then  be  maintained  with  a  voltage  in  the  range  of  15 
to  30  V.  Generally,  arc  welding  involves  using  a  metal  electrode  rod  and  attaching  the 
other  electrode  to  the  workpiece.  The  electrode  rod  either  melts  during  the  process 


100 


4     Joining  of  Metals 


(consumable  electrode)  and  provides  the  necessary  filler  metal  for  the  weld,  or  the 
electrode  does  not  melt  and  the  filler  metal  is  separately  provided. 

As  just  mentioned,  either  alternating  current  or  direct  current  can  be  used  in  arc 
welding,  although  each  has  its  distinct  advantages.  While  arc  stability  is  much  better 
with  alternating  current  than  with  direct  current,  the  ac  welding  equipment  is  far  less 
expensive,  more  compact  in  size,  and  simpler  to  operate.  A  further  advantage  of  ac  arc 
welding  is  the  high  efficiency  of  the  transformer  used,  which  goes  up  to  85  percent, 
whereas  the  efficiency  of  dc  welding  systems  usually  varies  between  only  30  and 
60  percent. 

In  dc  arc  welding,  the  degree  to  which  the  work  is  heated  can  be  regulated  by 
using  either  straight  or  reversed  polarity.  As  can  be  seen  in  Figure  4.24a  and  b,  the 
cathode  is  the  electrode  rod  and  the  anode  is  the  workpiece  in  straight  polarity  (DCSP), 
whereas  it  is  the  other  way  around  in  reversed  polarity  (DCRP).  When  using  DCSP, 
more  heat  is  concentrated  at  the  cathode  (the  electrode  rod)  than  at  the  anode  (the 
workpiece).  Therefore,  melting  and  deposition  rates  (of  consumable  electrodes)  are 
high,  but  penetration  in  the  workpiece  is  shallow  and  narrow.  Consequently,  DCSP  is 
recommended  when  welding  sheet  metal,  especially  at  higher  welding  speeds.  With 
DCRP,  heat  is  concentrated  at  the  cathode  (the  workpiece)  and  results  in  deeper  pene- 
tration for  a  given  welding  condition.  It  is,  therefore,  preferred  for  groove  welds  and 
similar  applications. 

During  the  welding  operation,  heat  is  generated  in  the  transformer  as  well  as  in 
other  elements  of  the  welding  circuit,  resulting  in  a  temperature  rise  that  may  cause 
damage  to  those  elements.  There  is,  therefore,  a  time  limitation  when  using  the  weld- 
ing equipment  at  a  given  amperage.  That  time  limitation  is  usually  referred  to  as  the 
rated  duty  cycle.  Consider  the  following  numerical  example.  A  power  supply  for  arc 
welding  rated  at  a  150- A  40-percent  duty  cycle  means  that  it  can  be  used  only  40 
percent  of  the  time  when  welding  at  150  A.  The  idle  or  unused  time  is  required  to 
allow  the  equipment  to  cool  down.  The  percentage  of  duty  cycles  at  currents  other  than 
the  rated  current  can  be  calculated  using  the  following  equation: 


%  duty  cycle 


-I 


rated  current  V 


\  load  current  / 


x  rated  duty  cycle 


(4.2) 


FIGURE    4.24 

Straight  and  reversed 
polarities  in  dc  arc 
welding:  (a)  dc  straight 
polarity  (DCSP);  (b)  dc 
reversed  polarity 
(DCRP) 


Electrode 
(cathode) 


w 


Electrode 
(anode) 


w 


Workpiece 
(anode) 


Workpiece 
(cathode) 


(a) 


(b) 


4.2  Welding 


101 


Therefore,  for  this  power  supply,  the  percentage  of  the  duty  cycle  at  100  A  is  as 
follows: 

%  duty  cycle  at  100  A  =  [  —  V  x  40%  =  90% 

There  are  various  types  of  arc  welding.  They  include  the  following  methods: 

1.  Shielded-metal  arc  welding  (SMAW) 

2.  Carbon  arc  welding  (CAW) 

3.  Flux-cored  arc  welding  (FCAW) 

4.  Stud  arc  welding  (SW) 

5.  Submerged  arc  welding  (SAW) 

6.  Gas-metal  arc  welding  (GMAW,  usually  called  MIG) 

7.  Gas-tungsten  arc  welding  (GTAW,  usually  called  TIG) 

8.  Plasma  arc  welding  (PAW) 

In  addition,  there  is  another  welding  process,  electroslag  welding  (EW),  which  is  not 
based  on  the  phenomenon  of  the  electric  arc  but,  nevertheless,  employs  equipment 
similar  to  that  used  in  gas-metal  arc.  flux-cored  arc,  or  submerged  arc  welding. 

1.  Shielded-metal  arc  welding.  Shielded-metal  arc  welding  (SMAW)  is  a  manual  arc 
welding  process  that  is  sometimes  referred  to  as  stick  welding.  The  source  of  heat 
for  welding  is  an  electric  arc  maintained  between  a  flux-covered,  consumable  metal 
electrode  and  the  workpiece.  As  can  be  seen  in  Figure  4.25,  which  indicates  the  op- 
erating principles  of  this  process,  shielding  of  the  electrode  tip,  weld  puddle,  and 
weld  area  in  the  base  metal  is  ensured  through  the  decomposition  of  the  flux  cov- 
ering. A  blanket  of  molten  slag  also  provides  shielding  for  the  molten-metal  pool. 
The  filler  metal  is  provided  mainly  by  the  metal  core  of  the  electrode  rod. 

Shielded-metal  arc  welding  can  be  used  for  joining  thin  and  thick  sheets  of 
plain-carbon  steels,  low-alloy  steels,  and  even  some  alloy  steels  and  cast  iron,  pro- 
vided that  the  electrode  is  properly  selected  and  also  that  preheating  and  postheat- 
ing  treatments  are  performed.  It  is  actually  the  most  commonly  used  welding 
process  and  has  found  widespread  application  in  steel  construction  and  shipbuild- 
ing. Nevertheless,  it  is  uneconomical  and/or  impossible  to  employ  shielded-metal 
arc  welding  to  join  some  alloys,  such  as  aluminum  alloys,  copper,  nickel,  copper- 
nickel  alloys,  and  low-melting-point  alloys  such  as  zinc,  tin,  and  magnesium  alloys. 


102 


4      Joining  of  Metals 


Another  clear  shortcoming  of  the  process  is  that  welding  must  be  stopped  each  time 
an  electrode  stick  is  consumed  to  allow  mounting  a  new  one.  This  results  in  idle 
time  and,  consequently,  a  drop  in  productivity. 

The  core  wires  of  electrodes  used  for  shielded-metal  arc  welding  have  many  dif- 
ferent compositions.  The  selection  of  a  particular  electrode  material  depends  upon 
the  application  for  which  it  is  going  to  be  used  and  the  kind  of  base  metal  to  be 
welded.  Consumable  electrodes  are  usually  coated  with  flux  but  can  also  be  un- 
coated.  The  metal  wire  can  have  a  diameter  of  up  to  1 5/  32  inch  ( 1 2  mm)  and  a  length 
of  about  18  inches  (450  mm).  Although  various  metals  are  used  as  wire  materials,  by 
far  the  most  commonly  used  electrode  materials  involve  low-carbon  steel  (for  weld- 
ing carbon  steels)  and  low-alloy  steel  (for  welding  alloy  steels).  Electrodes  can  be 
bare,  lightly  coated,  or  heavily  coated.  The  electrode  covering,  or  coating,  results  in 
better-quality  welds  as  it  improves  arc  stability,  produces  gas  shielding  to  prevent 
oxidation  and  nitrogen  contamination,  and  also  provides  slag,  which,  in  turn,  re- 
tards the  cooling  rate  of  the  weld's  fusion  zone.  Therefore,  electrode  coatings  are 
composed  of  substances  that  serve  these  purposes.  Table  4.1  indicates  the  composi- 
tion of  typical  electrode  coatings,  together  with  the  function  of  each  constituent. 

2.  Carbon  arc  welding.  In  carbon  arc  welding  (CAW),  nonconsumable  electrodes 
made  of  carbon  or  graphite  are  used.  Only  a  dc  power  supply  can  be  employed, 
and  the  electric  arc  is  established  either  between  a  single  carbon  electrode  and 
the  workpiece  (Bernardos  method)  or  between  two  carbon  electrodes  (independent 
arc  method).  In  both  cases,  no  shielding  is  provided.  A  filler  metal  may  be  used, 
especially  when  welding  sheets  with  thicknesses  more  than  1/8  inch  (3  mm).  The 
carbon  electrodes  have  diameters  ranging  from  3/8  to  1  inch  (10  to  25  mm)  and  are 
used  with  currents  that  range  from  200  to  600  A. 


TABLE    4.1 

The  constituents  of 
typical  electrode 
coatings  and  their 
functions 


Main  Function 

Constituent 

Percentage 

Gas  generating 

Starch 

Cellulose 

25-40% 

Calcium  carbonate 

Slag  forming 

Kaolin 
Titanium  dioxide 

Feldspar 

20-40% 

Asbestos 

Binding 

Sodium  silicate 
Potassium  silicate 

20-30% 

Deoxidizing 

Ferrosilicon 
Aluminum 

5-10% 

Arc  stabilizing 

Potassium  titanate 
Titanium  oxide 

5-10% 

Increasing  deposition  rate 

Iron  powder 

0-40% 

Improving  weld  strength 

Different  alloying  elements 

5-10% 

4.2  Welding 


103 


Carbon  arc  welding  is  not  commonly  used  in  industry.  Its  application  is  limited 
to  the  joining  of  thin  sheets  of  nonferrous  metals  and  to  brazing. 

3.  Flux-cored  arc  welding.  Flux-cored  arc  welding  (FCAW)  is  an  arc  welding  process 
in  which  the  consumable  electrode  takes  the  form  of  a  tubular,  flux-filled  wire,  that 
is  continuously  fed  from  a  spool.  Shielding  is  usually  provided  by  the  gases  evolv- 
ing during  the  combustion  and  decomposition  of  the  flux  contained  within  the 
tubular  wire.  The  process  is,  therefore,  sometimes  called  inner-shielded,  or  self- 
shielded,  arc  welding.  Additional  shielding  may  be  acquired  through  the  use  of  an 
auxiliary  shielding  gas,  such  as  carbon  dioxide,  argon,  or  both.  In  the  latter  case,  the 
process  is  a  combination  of  the  conventional  flux-cored  arc  welding  and  the  gas- 
metal  arc  welding  methods  and  is  referred  to  as  dual-shielded  arc  welding. 

Flux-cored  arc  welding  is  generally  applied  on  a  semiautomatic  basis,  but  it  can 
also  be  fully  automated.  In  that  case,  the  process  is  normally  used  to  weld  medium- 
to-thick  steel  plates  and  stainless  steel  sheets.  Figure  4.26  illustrates  the  operating 
principles  of  flux-cored  arc  welding. 

4.  Stud  arc  welding.  Stud  arc  welding  (SW)  is  a  special-purpose  arc  welding  process 
by  which  studs  are  welded  to  flat  surfaces.  This  facilitates  fastening  and  handling 
of  the  components  to  which  studs  are  joined  and  meanwhile  eliminates  the  drill- 
ing and  tapping  operations  that  would  have  been  required  to  achieve  the  same 
goal.  Only  dc  power  supplies  are  employed,  and  the  process  also  calls  for  the 
use  of  a  special  welding  gun  that  holds  the  stud  during  welding.  Figure  4.27  shows 
the  stages  involved  in  stud  arc  welding,  a  process  that  is  entirely  controlled  by  the 
timer  of  the  gun.  As  can  be  seen  in  the  figure,  shielding  is  accomplished  through  the 
use  of  a  ceramic  ferrule  that  surrounds  the  end  of  the  stud  during  the  process.  Stud 
arc  welding  requires  a  low  degree  of  welding  skill,  and  the  whole  welding  cycle 
usually  takes  less  than  a  second. 

5.  Submerged  arc  welding.  Submerged  arc  welding  (SAW)  is  a  fairly  new  automatic 
arc  welding  method  in  which  the  arc  and  the  weld  area  are  shielded  by  a  blanket  of 
a  fusible  granular  flux.  A  bare  electrode  is  used  and  is  continuously  fed  by  a  special 
mechanism  during  welding.  Figure  4.28  shows  the  operating  principles  of  sub- 
merged arc  welding.  As  can  be  seen  from  the  figure,  the  process  is  used  to  join  flat 
plates  in  the  horizontal  position  only.  This  limitation  is  imposed  by  the  nature  of  the 
flux  and  the  way  it  is  fed. 

As  is  the  case  with  previously  discussed  arc  welding  processes,  gases  evolve  as 
a  result  of  combustion  and  decomposition  of  the  flux,  due  to  the  high  temperature 


FIGURE    4.26 

Operating  principles  of 
flux-cored  arc  welding 


Solidified 
slag 


Electrode  (tube) 
Flux  core 


Base  metal 


104 


4      Joining  of  Metals 


FIGURE    4.27 

Stages  involved  in  stud 
arc  welding 


Stud 


Flux 


Light 
pressure 

■in 


IC^I 


wm 


Base  metal 


of  the  arc,  and  form  a  pocket,  or  gas  bubble,  around  the  arc.  As  Figure  4.29  shows, 
this  gas  bubble  is  sealed  from  the  arc  by  a  layer  of  molten  flux.  This  isolates  the  arc 
from  the  surrounding  atmosphere  and,  therefore,  ensures  proper  shielding. 

The  melting  temperature  of  the  flux  must  be  lower  than  that  of  the  base  metal. 
As  a  result,  the  flux  always  solidifies  after  the  metal,  thus  forming  an  insulating 
layer  over  the  solidifying  molten  metal  pool.  This  retards  the  solidification  of  the 
fused  metal  and,  therefore,  allows  the  slag  and  nonmetallic  inclusions  to  float  off 
the  molten  pool.  The  final  outcome  is  always  a  weld  that  is  free  of  nonmetallic  in- 
clusions and  entrapped  gases  and  has  a  homogeneous  chemical  composition.  The 
flux  should  also  be  selected  to  ensure  proper  deoxidizing  of  the  fused  metal  and 


FIGURE    4.28 

Operating  principles  of 
submerged  arc  welding 


FIGURE    4.29 

The  mechanics  of 
shielding  in  submerged 
arc  welding 


Hose  from  hopper 

to  supply 

granulated  flux 


Direction  of  weld 


Base  meta 


Solidified 
slag 


Molten 
flux 


Granulated 
flux 


Solidified 
weld  metal 

Base  metal 


Gas 
bubble 


Molten 
metal 


4.2  Welding 


105 


should  contain  additives  that  make  up  for  the  elements  burned  and  lost  during  the 
welding  process. 

Electric  currents  commonly  used  with  submerged  arc  welding  range  between 
3000  and  4000  A.  Consequently,  the  arc  obtained  is  extremely  powerful  and  is  ca- 
pable of  producing  a  large  molten-metal  pool  as  well  as  achieving  deeper  penetra- 
tion. Other  advantages  of  this  process  include  its  high  welding  rate,  which  is  five  to 
ten  times  that  produced  by  shielded-metal  arc  welding,  and  the  high  quality  of  the 
welds  obtained. 

6.  Gas-metal  arc  welding.  The  GMAW  process  is  commonly  called  metal-inert-gas 
(MIG)  welding.  It  employs  an  electric  arc  between  a  solid,  continuous,  consumable 
electrode  and  the  workpiece.  As  can  be  seen  in  Figure  4.30,  shielding  is  obtained 
by  pumping  a  stream  of  chemically  inert  gas,  such  as  argon  or  helium,  around  the 
arc  to  prevent  the  surrounding  atmosphere  from  contaminating  the  molten  metal. 
(The  electrode  is  bare,  and  no  flux  is  added.)  Dry  carbon  dioxide  can  sometimes  be 
employed  as  a  shielding  gas,  yielding  fairly  good  results. 

Gas-metal  arc  welding  is  generally  a  semiautomatic  process.  However,  it  can 
also  be  applied  automatically  by  machine.  In  fact,  welding  robots  and  numerically 
controlled  MIG  welding  machines  have  gained  widespread  industrial  application. 
The  gas-metal  arc  welding  process  can  be  used  to  weld  thin  sheets  as  well  as  rela- 
tively thick  plates  in  all  positions,  and  the  process  is  particularly  popular  when 
welding  nonferrous  metals  such  as  aluminum,  magnesium,  and  titanium  alloys.  The 
process  is  also  used  for  welding  stainless  steel  and  critical  steel  parts. 

The  penetration  for  gas-metal  arc  welding  is  controlled  by  adopting  DCRP  and 
adjusting  the  current  density.  The  higher  the  current  density  is,  the  greater  the  pen- 
etration is.  The  kind  of  shielding  gas  used  also  has  some  effect  on  the  penetration. 
For  instance,  helium  gives  the  maximum  penetration;  carbon  dioxide,  the  least; 
argon,  intermediate  penetration.  Thus,  it  is  clear  that  higher  current  densities  and  the 


FIGURE    4.30 

Operating  principles  of 
gas-metal  arc  welding 


Spool 

for  feeding 

electrode  wire 


Inert  gas  cylinder 
for  providing 
shielding  gas 


Base  metal 


106 


Joining  of  Metals 


appropriate  shielding  gas  can  be  employed  in  welding  thick  plates,  provided  that  the 
edges  of  these  plates  are  properly  prepared. 

The  electrode  wires  used  for  MIG  welding  must  possess  close  dimensional  tol- 
erances and  a  consistent  chemical  composition  appropriate  for  the  desired  applica- 
tion. The  wire  diameter  varies  between  0.02  and  0.125  inch  (0.5  and  3  mm). 
Usually,  MIG  wire  electrodes  are  coated  with  a  very  thin  layer  of  copper  to  protect 
them  during  storage.  The  electrode  wire  is  available  in  the  form  of  a  spool  weigh- 
ing from  2V2  to  750  pounds  (1  to  350  kg).  As  you  may  expect,  the  selection  of  the 
composition  of  the  electrode  wire  for  a  given  material  depends  upon  other  factors, 
such  as  the  kind  of  shielding  gas  used,  the  conditions  of  the  metal  being  welded 
(i.e.,  whether  there  is  an  oxide  film,  grease,  or  contaminants),  and,  finally,  the  re- 
quired properties  of  the  weldment. 

Gas-tungsten  arc  welding.  Gas-tungsten  arc  welding  (GTAW),  which  is  usually 
called  tungsten-inert-gas  (TIG)  welding,  is  an  arc  welding  process  that  employs  the 
heat  generated  by  an  electric  arc  between  a  nonconsumable  tungsten  electrode  and 
the  workpiece.  Figure  4.31  illustrates  the  operating  principles  of  this  process.  As 
can  be  seen,  a  filler  rod  may  (or  may  not)  be  fed  to  the  arc  zone.  The  electrode,  arc, 
weld  puddle,  and  adjacent  areas  of  the  base  metal  are  shielded  by  a  stream  of  either 
argon  or  helium  to  prevent  any  contamination  from  the  atmosphere.  TIG  welding  is 
normally  applied  manually  and  requires  a  relatively  high  degree  of  welder  skill.  It 
can  also  be  fully  automated,  in  which  case  the  equipment  used  drives  the  welding 
torch  at  a  preprogrammed  path  and  speed,  adjusts  the  arc  voltage,  and  starts  and 
stops  it. 

Gas-tungsten  arc  welding  is  capable  of  welding  nonferrous  and  exotic  metals 
in  all  positions.  The  list  of  metals  that  can  be  readily  welded  by  this  process  is 
long  and  includes  alloy  steels,  stainless  steels,  heat-resisting  alloys,  refractory  met- 
als, aluminum  alloys,  magnesium  alloys,  titanium  alloys,  copper  and  nickel  alloys, 
and  steel  coated  with  low-melting-point  alloys.  The  process  is  recommended  for 


FIGURE    4.31 

Operating  principles  of 
gas-tungsten  arc 
welding 


Solidified 
weld  metal 


Tungsten 
Torch         electrode 

Shielding 
gas 


Handle 


Filler  rod 


The  welding  torch 


Base  metal 

Electrode-protecting 
cap 


Tungsten 
electrode 


4.2  Welding 


107 


welding  very  thin  sheets,  as  thin  as  0.005  inch  (about  0.125  mm),  for  the  root  and 
hot  pass  on  tubing  and  pipes,  and  wherever  smooth,  clean  welds  are  required  (e.g., 
in  food-processing  equipment).  Ultrahigh-quality  welds  can  be  obtained  in  the  nu- 
clear, rocketry,  and  submarine  industries  by  employing  a  modified  version  of  TIG 
welding  that  involves  placing  carefully  selected  and  prepared  inserts  in  the  gap  be- 
tween the  sections  to  be  joined  and  then  completely  fusing  the  inserts  together  with 
the  edges  of  base  metal  using  a  TIG  torch. 

All  three  types  of  current  supplies  (i.e.,  ac,  DCSP,  and  DCRP)  can  be  used  with 
gas-tungsten  arc  welding,  depending  upon  the  metal  to  be  welded.  Thin  sheets  of 
aluminum  or  magnesium  alloys  are  best  welded  by  using  DCRP,  which  prevents 
burn-throughs,  as  previously  explained.  Nevertheless,  it  is  recommended  that  an  ac 
power  supply  be  used  when  welding  normal  sheets  of  aluminum  and  magnesium. 
DCSP  is  best  suited  for  welding  high-melting-point  alloys  such  as  alloy  steels, 
stainless  steels,  heat-resisting  alloys,  copper  alloys,  nickel  alloys,  and  titanium.  In 
addition  to  these  considerations,  DCRP  is  also  helpful  in  removing  surface  oxide 
films  due  to  its  cleaning  action  (the  impacting  of  ions  onto  the  surface  like  a  grit 
blasting). 

8.  Plasma  arc  welding.  Figure  4.32  is  a  sketch  of  the  torch  employed  in  plasma  arc 
welding  (PAW).  The  electric  arc  can  take  either  of  two  forms:  a  transferred  arc  that 
is  a  constricted  arc  between  a  tungsten  electrode  and  the  workpiece  or  a  nontrans- 
ferred  arc  between  the  electrode  and  the  constricting  nozzle.  The  gas  flowing 
around  the  arc  heats  up  to  extremely  high  temperatures  like  60,000°F  (33,000°C) 
and  becomes,  therefore,  ionized  and  electrically  conductive;  it  is  then  referred  to  as 
plasma.  The  main  shielding  is  obtained  from  the  hot  ionized  gas  emerging  from  the 
nozzle.  Additional  inert-gas  shielding  can  be  used  when  high-quality  welds  are  re- 
quired. In  fact,  plasma  arc  welding  can  be  employed  to  join  almost  all  metals  in  all 
positions,  although  it  is  usually  applied  to  thinner  metals.  Generally,  the  process  is 
applied  manually  and  requires  some  degree  of  welder  skill;  however,  the  process  is 
sometimes  automated  in  order  to  increase  productivity. 

Electroslag  welding.    Electroslag  welding  (ESW),  which  was  developed  by  the  Rus- 
sians, is  not  an  arc  welding  process  but  requires  the  use  of  equipment  similar  to  that 


FIGURE    4.32 

The  torch  employed  in 
plasma  arc  welding 


Nozzle 


Tungsten 
electrode 


^J^E^J^ 


Shielding 
gas 


Workpiece 


4      Joining  of  Metals 

used  in  arc  welding.  Although  an  electric  arc  is  used  to  start  the  process,  heat  is  con- 
tinuously generated  as  a  result  of  the  current  flow  between  the  electrode  (or  electrodes) 
and  the  base  metal  through  a  pool  of  molten  slag  (flux).  As  we  will  see  later,  the 
molten-slag  pool  also  serves  as  a  protective  cover  for  the  fused-metal  pool. 

The  electroslag  welding  process  is  shown  in  Figure  4.33.  As  can  be  seen  in  the  fig- 
ure, the  parts  to  be  joined  are  set  in  the  vertical  position,  with  a  gap  of  1/2  to  l!/2 
inches  (12  to  37  mm)  between  their  edges.  (The  gap  is  dependent  upon  the  thickness 
of  the  parts.)  The  welding  electrode  (or  electrodes)  and  the  flux  are  fed  automatically 
into  the  gap,  and  an  arc  is  established  between  the  electrodes  and  the  steel  backing 
plate  to  provide  the  initial  molten-metal  and  slag  pools.  Next,  the  electrical  resistivity 
of  the  molten  slag  continuously  produces  the  heat  necessary  to  fuse  the  flux  and  the 
filler  and  the  base  metals.  Water-cooled  copper  plates  travel  upward  along  the  joint, 
thus  serving  as  dams  and  cooling  the  fused  metal  in  the  cavity  to  form  the  weld. 

Electroslag  welding  is  very  advantageous  in  joining  very  thick  parts  together  in  a 
single  pass  without  any  need  for  beveling  the  edges  of  those  parts.  Therefore,  the 
process  is  widely  used  in  industries  that  fabricate  beds  and  frames  for  heavy  machin- 
ery, drums,  boilers,  and  the  like. 

Gas  welding.  Gas  welding  refers  to  a  group  of  oxyfuel  gas  processes  in  which  the 
edges  of  the  parts  to  be  welded  are  fused  together  by  heating  them  with  a  flame  ob- 
tained from  the  combustion  of  a  gas  (such  as  acetylene)  in  a  stream  of  oxygen.  A  filler 
metal  is  often  introduced  into  the  flame  to  melt  and,  together  with  the  base  metal,  form 
the  weld  puddle.  Gas  welding  is  usually  applied  manually  and  requires  good  welding 
skill.  Common  industrial  applications  involve  welding  thin-to-medium  sheets  and  sec- 
tions of  steels  and  nonferrous  metals  in  all  positions.  Gas  welding  is  also  widely  used 
in  repair  work  and  in  restoring  cracked  or  broken  components. 

The  fuel  gases  used  for  producing  the  flame  during  the  different  gas  welding 
processes  include  acetylene,  hydrogen,  natural  gas  (94  percent  methane),  petroleum 
gas,  and  vaporized  gasoline  and  kerosene.  However,  acetylene  is  the  most  commonly 
used  gas  for  gas  welding  because  it  can  provide  a  flame  temperature  of  about  5700°F 
(3150°C).  Unfortunately,  acetylene  is  ignited  at  a  temperature  as  low  as  790°F  (420°C) 
and  becomes  explosive  in  nature  at  pressures  exceeding  1 .75  atmospheres.  Therefore, 
it  is  stored  in  metal  cylinders,  in  which  it  is  dissolved  in  acetone  under  a  pressure  of 


FIGURE    4.33 

The  electroslag  welding 
process 


Guiding 
tubes 


Base  metal 


Molten  slag 
Molten  metal 


Base  plate 


4.2  Welding 


109 


about  19  atmospheres.  For  more  safety,  acetylene  cylinders  are  also  filled  with  a 
porous  filler  (such  as  charcoal)  in  order  to  form  a  system  of  capillary  vessels  that  are 
then  saturated  with  the  solution  of  acetylene  in  acetone. 

The  oxygen  required  for  the  gas  welding  process  is  stored  in  steel  cylinders  in  the 
liquid  state  under  a  pressure  of  about  150  atmospheres.  It  is  usually  prepared  in  spe- 
cial plants  by  liquefying  air  and  then  separating  the  oxygen  from  the  nitrogen. 

The  equipment  required  in  gas  welding,  as  shown  in  Figure  4.34,  includes  oxygen 
and  acetylene  cylinders,  regulators,  and  the  welding  torch.  The  regulators  serve  to  re- 
duce the  pressure  of  the  gas  in  the  cylinder  to  the  desired  working  value  and  keep  it 
that  way  throughout  the  welding  process.  Thus,  the  proportion  of  the  two  gases  is  con- 
trolled, which  determines  the  characteristics  of  the  flame.  Next,  the  welding  torch 
serves  to  mix  the  oxygen  and  the  acetylene  together  and  discharges  the  mixture  out  at 
the  tip,  where  combustion  takes  place. 

Depending  upon  the  ratio  of  oxygen  to  acetylene,  three  types  of  flames  can  be  ob- 
tained: neutral,  reducing,  and  oxidizing.  Figure  4.35  is  a  sketch  of  a  typical  oxyacety- 
lene  welding  flame.  As  can  be  seen,  the  welding  flame  consists  of  three  zones:  the 
inner  luminous  cone  at  the  tip  of  the  torch,  the  reducing  zone,  and  the  oxidizing  zone. 

The  first  zone,  the  luminous  cone,  consists  of  partially  decomposed  acetylene  as  a 
result  of  the  following  reaction: 

C2H2  ->  2C  +  H2 

The  carbon  particles  obtained  are  incandescent  and  are  responsible  for  the  white  lumi- 
nescence of  that  brightest  part  of  the  flame.  Those  carbon  particles  are  partly  oxidized 
in  the  second  zone,  the  reducing  zone,  yielding  carbon  monoxide  and  a  large  amount 
of  heat  that  brings  the  temperature  up  to  about  5400°F  (3000°C).  Gases  like  hydrogen 
and  carbon  monoxide  are  capable  of  reducing  oxides.  Next,  complete  combustion  of 
those  gases  yields  carbon  dioxide  and  water  vapor  that  together  with  the  excess  oxy- 
gen (if  any)  result  in  the  third  zone,  the  oxidizing  zone.  Those  gases,  however,  form  a 
shield  that  prevents  the  atmosphere  from  coming  in  contact  with  the  molten-metal 
pool. 

As  can  be  expected,  the  extent  (as  well  as  the  appearance)  of  each  of  the  zones 
depends  upon  the  type  of  flame  (i.e.,  the  oxygen-to-acetylene  ratio).  When  the  ratio 
is  about  1,  the  flame  is  neutral  and  distinctively  has  the  three  zones  just  outlined.  If 
the  oxygen-to-acetylene  ratio  is  less  than  1,  a  reducing,  or  carbonizing,  flame  is  ob- 
tained. In  this  case,  the  luminous  cone  is  longer  than  that  obtained  with  the  neutral 


FIGURE    4.34 

The  equipment  required 
in  gas  welding 


Hoses 


Oxygen 


110 


4      Joining  of  Metals 


FIGURE    4.35 

A  sketch  of  a  typical 
oxyacetylene  welding 
flame 


flame,  and  the  outline  of  the  flame  is  not  sharp.  This  type  of  flame  is  employed  in 
welding  cast  iron  and  in  hard-surfacing  with  high-speed  steel  and  cemented  carbides. 
The  third  type  of  flame,  the  oxidizing  flame,  is  obtained  when  the  oxygen-to-acety- 
lene ratio  is  higher  than  1.  In  this  case,  the  luminous  cone  is  shorter  than  that  ob- 
tained with  the  neutral  flame,  and  the  flame  becomes  light  blue  in  color.  The 
oxidizing  flame  is  employed  in  welding  brass,  bronze,  and  other  metals  that  have 
great  affinity  to  hydrogen. 

Another  method  that  utilizes  the  heat  generated  as  a  result  of  the  combustion  of 
a  fuel  gas  is  known  as  pressure-gas  welding.  As  the  name  suggests,  this  is  actually  a 
pressure  welding  process  in  which  the  abutting  edges  to  be  welded  are  heated  with 
an  oxyacetylene  flame  to  attain  a  plastic  state;  then,  coalescence  is  achieved  by  ap- 
plying the  appropriately  high  pressure.  In  order  to  ensure  uniform  heating  of  the  sec- 
tions, a  multiple-flame  torch  that  surrounds  the  sections  is  used.  The  shape  of  that 
torch  is  dependent  upon  the  outer  contour  of  the  sections  to  be  welded,  and  the  torch 
is  usually  made  to  oscillate  along  its  axis.  Upsetting  is  accomplished  by  a  special 
pressure  mechanism.  This  method  is  sometimes  used  for  joining  pipeline  mains,  rails, 
and  the  like. 

Electron-beam  welding.  Electron-beam  welding  (EBW)  was  developed  by  Dr. 
Jacques  Stohr  (CEA-France,  the  atomic  energy  commission)  in  1957  to  solve  a  prob- 
lem in  the  manufacturing  of  fuel  elements  for  atomic  power  generators.  The  process  is 
based  upon  the  conversion  of  the  kinetic  energy  of  a  high-velocity,  intense  beam  of 
electrons  into  thermal  energy  as  the  accelerated  electrons  impact  on  the  joint  to  be 
welded.  The  generated  heat  then  fuses  the  interfacing  surfaces  and  produces  the  de- 
sired coalescence. 

Figure  4.36  shows  the  basic  elements  and  working  principles  of  an  electron-beam 
welding  system.  The  system  consists  of  an  electron-beam  gun  (simply  an  electron 
emitter  such  as  a  hot  filament)  that  is  electrically  placed  at  a  negative  potential  with  re- 
spect to  an  anode  and  that  together  with  the  workpiece  is  earth-grounded.  A  focus  coil 
(i.e.,  an  electromagnetic  lens)  is  located  slightly  below  the  anode  in  order  to  bring  the 
electron  beam  into  focus  upon  the  work.  This  is  achieved  by  adjusting  the  current  of 
the  focus  coil.  Additional  electromagnetic  coils  are  provided  to  deflect  the  beam  from 
its  neutral  axis  as  required.  Because  the  electrons  impacting  the  work  travel  at  an  ultra- 
high velocity,  the  process  should  be  carried  out  in  a  vacuum  in  order  to  eliminate  any 
resistance  to  the  traveling  electrons.  Pressures  on  the  order  of  10  torr  (1  atmosphere  = 
760  torr)  are  commonly  employed,  although  pressures  up  to  almost  atmospheric  can  be 
used.  Nevertheless,  it  must  be  noted  that  the  higher  the  pressure  is,  the  wider  and  more 
dispersed  the  electron  beam  becomes,  and  the  lower  the  energy  density  is.  (Energy 
density  is  the  number  of  kilowatts  per  unit  area  of  the  spot  being  welded.) 


4.2  Welding 


111 


FIGURE    4.36 

The  basic  elements  and 
working  principles  of  an 
electron-beam  welding 
system 


Filament 
current 
control 


Electron 
beam 


Accelerating 
~      voltage 
+      control 


Electron-beam  welding  machines  can  be  divided  into  two  groups:  low-voltage  and 
high-voltage  machines.  Low-voltage  machines  are  those  operating  at  accelerating  volt- 
ages up  to  60  kV,  whereas  high-voltage  machines  operate  at  voltages  up  to  200  kV.  Al- 
though each  of  these  two  types  has  its  own  merits,  the  main  consideration  should  be 
the  beam-power  density,  which  is,  in  turn,  dependent  upon  the  beam  power  and  the 
(focused)  spot  size.  In  the  early  days  of  electron-beam  welding,  machines  were  usually 
built  to  have  a  rating  of  7.5  kW  and  less.  Today,  a  continuous-duty  rating  of  60  kW  is 
quite  common,  and  the  trend  is  toward  still  higher  ratings. 

There  are  several  advantages  to  the  electron-beam  welding  process.  They  include 
the  following  five: 

1.  Because  of  the  high  intensity  of  the  electron  beam  used,  the  welds  obtained  are 
much  narrower,  and  the  penetration  in  a  single  pass  is  much  greater  than  that  ob- 
tained by  conventional  fusion  welding  processes. 

2.  The  high  intensity  of  the  electron  beam  can  also  develop  and  maintain  a  bore- 
hole in  the  workpiece,  thus  yielding  a  parallel-sided  weld  with  a  very  narrow  heat- 
affected  zone.  As  a  consequence,  the  welds  produced  by  this  method  have  almost 
no  distortion,  have  minimum  shrinkage,  and  are  stronger  than  welds  produced  by 
conventional  fusion  welding  processes. 

3.  Because  parallel-sided  welds  are  obtained  by  this  process,  there  is  no  need  for  edge 
preparation  of  the  workpieces  (such  as  V-  or  J-grooves).  Square  butt-type  joints  are 
commonly  produced  by  electron-beam  welding. 

4.  High  welding  speeds  can  be  obtained  with  this  process.  Speeds  up  to  200  inches  per 
minute  (0.09  m/s)  are  common,  resulting  in  higher  productivity. 

5.  Because  the  process  is  usually  performed  in  a  vacuum  chamber  at  pressures  on  the 
order  of  10  torr,  the  resulting  weld  is  excellent,  is  metallurgically  clean,  and  has 
an  extremely  low  level  of  atmospheric  contamination.  Therefore,  electron-beam 


112 


4      Joining  of  Metals 


welding  is  especially  attractive  for  joining  refractory  metals  whose  properties  are 
detrimentally  affected  by  even  low  levels  of  contamination. 

Because  of  the  ultrahigh  quality  of  the  joints  produced  by  electron-beam  welding, 
the  process  has  found  widespread  use  in  the  atomic  power,  jet  engine,  aircraft,  and 
aerospace  industries.  Nevertheless,  the  time  required  to  vacuum  the  chamber  before 
each  welding  operation  results  in  reduced  productivity,  and,  therefore,  the  high  cost  of 
the  electron-beam  welding  equipment  is  not  easily  justified.  This  apparently  kept  the 
process  from  being  applied  in  other  industries  until  it  was  automated.  Today,  electron- 
beam  welding  is  becoming  popular  for  joining  automotive  parts  such  as  gear  clusters, 
valves,  clutch  plates,  and  transmission  components. 

Laser-beam  welding.  The  term  laser  stands  for  light  amplified  by  stimulated  emission 
of  radiation.  It  is,  therefore,  easy  to  see  that  a  laser  beam  is  actually  a  controlled,  in- 
tense, highly  collimated,  and  coherent  beam  of  light.  In  fact,  a  laser  beam  proved  to  be 
a  unique  source  of  high-intensity  energy  that  can  be  used  in  fusing  metals  to  produce 
welded  joints  having  very  high  strength. 

Figure  4.37  shows  the  working  principles  of  laser-beam  welding  (LBW).  In  this 
laser  system,  energy  is  pumped  into  a  laser  medium  to  cause  it  to  fluoresce.  This  fluo- 
rescence, which  has  a  single  wavelength  or  color,  is  trapped  in  the  laser  medium  (laser 
tube)  between  two  mirrors.  Consequently,  it  is  reflected  back  and  forth  in  an  optical 
resonator  path,  resulting  in  more  fluorescence,  which  amplifies  the  intensity  of  the 
light  beam.  The  amplified  light  (i.e.,  the  laser  beam)  finds  its  way  out  through  the 
partly  transparent  mirror,  which  is  called  the  output  mirror.  The  laser  medium  can  be 
a  solid,  such  as  a  crystal  made  of  yttrium  aluminum  garnet  (YAG).  It  can  also  be  a  gas, 
such  as  carbon  dioxide,  helium,  or  neon.  In  the  latter  case,  the  pumping  energy  input 
is  usually  introduced  directly  by  electric  current  flow. 

Let  us  now  consider  the  mechanics  of  laser-beam  welding.  The  energy  intensity  of 
a  laser  beam  is  not  high  enough  to  fuse  a  metal  such  as  steel  or  copper.  Therefore,  it 
must  be  focused  by  a  highly  transparent  lens  at  a  very  tiny  spot,  0.01  inch  (0.25  mm) 
in  diameter,  in  order  to  increase  the  intensity  of  energy  up  to  a  level  of  10  million  W 
per  square  inch  (15,500  W/mm")  at  the  focal  point.  The  impacting  laser  energy  is 


FIGURE    4.37 

The  working  principles 
of  a  laser-beam  welding 
system 


Pumping 
energy  input 


Laser  media 


V 


t 


Totally 

reflective 

mirror 


Output  mirror 
(partially  transparent) 


Random 

fluorescence 

(losses) 


4.2  Welding  113 

converted  into  heat  as  it  strikes  the  surface  of  a  metal,  causing  instantaneous  fusion  of 
the  metal  at  the  focal  point.  Next,  a  cylindrical  cavity,  known  as  a  keyhole,  that  is  full 
of  vaporized,  ionized  metallic  gas  is  formed  and  is  surrounded  by  a  narrow  region  of 
molten  metal.  As  the  beam  moves  relative  to  the  workpiece,  the  molten  metal  fills  be- 
hind the  keyhole  and  subsequently  cools  and  solidifies  to  form  the  weld.  It  is  worth 
mentioning  that  a  stream  of  a  cooling  (and  shielding)  gas  should  surround  the  laser 
beam  to  protect  the  focusing  lens  from  vaporized  metal.  Usually,  argon  is  used  for  this 
purpose  because  of  its  low  cost,  although  helium  is  actually  the  best  cooling  gas. 

In  spite  of  the  high  initial  capital  cost  required,  laser-beam  welding  has  gained 
widespread  industrial  application  because  of  several  advantages  that  the  process  pos- 
sesses. Among  these  advantages  are  the  following  six: 

1.  Based  on  the  preceding  discussion  of  the  mechanics  of  laser-beam  welding,  we 
would  always  expect  to  have  a  very  narrow  heat-affected  zone  with  this  welding 
method.  Consequently,  the  chemical,  physical,  and  mechanical  properties  of  the 
base  metal  are  not  altered,  thus  eliminating  the  need  for  any  postwelding  heat 
treatment. 

2.  The  ultrahigh  intensity  of  energy  of  the  laser  beam  at  the  focal  point  allows  metals 
having  high  melting  points  (refractory  metals)  to  be  welded. 

3.  The  process  can  be  successfully  used  to  weld  both  nonconductive  as  well  as  mag- 
netic materials  that  are  almost  impossible  to  join  even  with  electron-beam  welding. 

4.  The  laser  beam  can  be  focused  into  a  chamber  through  highly  transparent  windows, 
thus  rendering  laser-beam  welding  suitable  for  joining  radioactive  materials  and  for 
welding  under  sterilized  conditions. 

5.  The  process  can  be  used  for  welding  some  materials  that  have  always  been  consid- 
ered unweldable. 

6.  The  process  can  be  easily  automated.  Numerically  controlled  laser-beam  welding 
systems  are  quite  common  and  are  capable  of  welding  along  a  complex  contour. 

Since  the  Apollo  project,  laser-beam  welding  has  become  popular  in  the  aerospace 
industry.  Today,  the  process  is  mainly  employed  for  joining  exotic  metals  such  as  tita- 
nium, tantalum,  zirconium,  columbium,  and  tungsten.  The  process  is  especially  advan- 
tageous for  making  miniature  joints  as  in  tiny  pacemaker  cans,  integrated-circuit 
packs,  camera  parts,  and  batteries  for  digital  watches.  Nevertheless,  laser-beam  weld- 
ing is  not  recommended  for  joining  brass,  zinc,  silver,  gold,  or  galvanized  steel. 

Welding  Defects 

In  fusion  welding  processes,  considerable  thermal  stresses  develop  during  heating  and 
subsequent  cooling  of  the  workpiece,  especially  with  those  processes  that  result  in 
large  heat-affected  zones.  Also,  metallurgical  changes  and  structural  transformations 
take  place  in  the  weld  puddle  as  well  as  in  the  heat-affected  zone,  and  these  may  be 
accompanied  by  changes  in  the  volume.  Therefore,  if  no  precautions  are  taken,  defects 


114  4      Joining  of  Metals 


that  are  damaging  to  the  function  of  the  weldment  may  be  generated.  It  is  the  com- 
bined duty  of  the  manufacturing  engineer,  the  welder,  and  the  inspector  to  make  sure 
that  all  weldments  are  free  from  all  kinds  of  defects.  Following  is  a  brief  survey  of  the 
common  kinds  of  welding  defects. 

Distortion.  Distortion,  warping,  and  buckling  of  the  welded  parts  are  welding  defects 
involving  deformation  (which  can  be  plastic)  of  the  structures  as  a  result  of  residual 
stresses.  They  come  as  a  result  of  restraining  the  free  movement  of  some  parts  or  mem- 
bers of  the  welded  structure.  They  can  also  result  from  nonuniform  expansion  and 
shrinkage  of  the  metal  in  the  weld  area  as  a  consequence  of  uneven  heating  and  cool- 
ing. Although  it  is  possible  to  predict  the  magnitude  of  the  residual  stresses  in  some 
simple  cases  (e.g.,  butt  welding  of  two  plates),  an  analysis  to  predict  the  magnitude 
of  these  stresses  and  to  eliminate  distortion  in  the  common  case  of  a  welded  three- 
dimensional  structure  is  extremely  complicated.  Nevertheless,  here  are  some  recom- 
mendations and  guidelines  to  follow  to  eliminate  distortion: 

1.  Preheat  the  workpieces  to  a  temperature  dependent  on  the  properties  of  the  base 
metal  in  order  to  reduce  the  temperature  gradient. 

2.  Clamp  the  various  elements  (to  be  welded)  in  a  specially  designed  rigid  welding 
fixture.  Although  no  distortion  occurs  with  this  method,  there  are  always  inher- 
ent internal  stresses.  The  internal  stresses  can  be  eliminated  by  subsequent 
stress-relieving  heat  treatment. 

3.  Sometimes,  it  is  adequate  just  to  tack-weld  the  elements  securely  in  the  right  posi- 
tion (relative  to  each  other)  before  actual-strength  welds  are  applied.  It  is  also  ad- 
visable to  start  by  welding  the  section  least  subject  to  distortion  first  in  order  to 
form  a  rigid  skeleton  that  contributes  to  the  balance  of  assembly. 

4.  Create  a  rational  design  of  weldments  (e.g.,  apply  braces  to  sections  most  likely  to 
distort). 

Porosity.  Porosity  can  take  the  form  of  elongated  blowholes  in  the  weld  puddle, 
which  is  known  as  wormhole  porosity,  or  of  scattered  tiny  spherical  holes.  In  both 
cases,  porosity  is  due  mainly  to  either  the  evolution  of  gases  during  welding  or  the  re- 
lease of  gases  during  solidification  as  a  result  of  their  decreasing  solubility  in  the  so- 
lidifying metal.  Excess  sulfur  or  sulfide  inclusions  in  steels  are  major  contributors  to 
porosity  because  they  generate  gases  that  are  often  entrapped  in  the  molten  metal. 
Other  causes  of  porosity  include  the  presence  of  hydrogen  (remember  the  problem 
caused  by  hydrogen  in  casting),  contamination  of  the  joint,  and  contaminants  in  the 
flux.  Porosity  can  be  eliminated  by  maintaining  clean  workpiece  surfaces,  by  properly 
conditioning  the  electrodes,  by  reducing  welding  speed,  by  eliminating  any  moisture 
on  workpieces,  and,  most  importantly,  by  avoiding  the  use  of  a  base  metal  containing 
sulfur  or  electrodes  with  traces  of  hydrogen. 

Cracks.  Welding  cracks  can  be  divided  into  two  main  groups:  fusion  zone  cracks 
and  heat-affected  zone  cracks.  The  first  group  includes  longitudinal  and  transverse 
cracks  as  well  as  cracks  appearing  at  the  root  of  the  weld  bead.  This  type  of  cracking 


4.2  Welding 


115 


is  sometimes  called  hot  cracking  because  it  occurs  at  elevated  temperatures  just 
after  the  molten  metal  starts  to  solidify.  It  is  especially  prevalent  in  ferrous  alloys  with 
high  percentages  of  sulfur  and  phosphorus  and  in  alloys  having  large  solidification 
ranges. 

The  second  type  of  cracking,  heat-affected  zone  cracks,  is  also  called  cold  crack- 
ing. This  defect  is  actually  due  to  aggravation  by  excessive  brittleness  of  the  heat- 
affected  zone  that  can  be  caused  by  hydrogen  embrittlement  or  by  martensite 
formation  as  a  result  of  rapid  cooling,  especially  in  high-carbon  and  alloy-steel  welded 
joints.  (Remember  the  effect  of  alloying  elements  on  the  TTT  curve;  they  shift  it  to  the 
right,  thus  decreasing  the  critical  cooling  rate.)  Cold  cracks  can  be  eliminated  by  using 
a  minimum  potential  source  of  hydrogen  and  by  controlling  the  cooling  rate  of  the 
welded  joint  to  keep  it  at  a  minimum  (e.g.,  keep  joints  in  a  furnace  after  welding  or 
embed  them  in  sand). 

The  use  of  multiple  passes  in  welding  can  sometimes  eliminate  the  need  for 
prewelding  or  postwelding  heat  treatment.  Each  pass  would  provide  a  sort  of  preheat- 
ing for  the  pass  to  follow.  This  technique  is  often  effective  in  the  prevention  of  weld 
cracks. 

Slag  inclusions.  Slag  entrapment  in  the  weld  zone  can  occur  in  single-pass  as  well  as 
in  multipass  welds.  In  single-pass  arc  welding,  slag  inclusions  are  caused  by  improper 
manipulation  of  the  electrode  and/or  factors  such  as  too  high  a  viscosity  of  the  molten 
metal  or  too  rapid  solidification.  Some  slag  pushed  ahead  of  the  arc  is  drawn  down  by 
turbulence  into  the  molten-metal  pool,  where  it  becomes  entrapped  in  the  solidifying 
weld  metal.  In  multipass  welds,  slag  inclusions  are  caused  by  improper  removal  of  the 
slag  blanket  after  each  pass. 

Lack  of  fusion.  Lack  of  fusion,  shown  in  Figure  4.38,  can  result  from  a  number  of 
causes.  These  include  inadequate  energy  input,  which  leads  to  insufficient  temperature 
rise;  improper  electrode  manipulation;  and  failure  to  remove  oxide  films  and  clean  the 
weld  area  prior  to  welding. 

Lack  of  penetration.  Lack  of  penetration,  shown  in  Figure  4.39,  is  due  to  a  low  en- 
ergy input,  the  wrong  polarity,  or  a  high  welding  speed. 

Undercutting.  Undercutting,  shown  in  Figure  4.40,  is  a  result  of  a  high  energy  input 
(excessive  current  in  arc  welding),  which,  in  turn,  causes  the  formation  of  a  recess.  As 
we  know,  such  sharp  changes  in  the  weld  contour  act  as  stress  raisers  and  often  cause 
premature  failure. 


FIGURE    4.38 

Lack  of  fusion 


FIGURE    4.39 

Lack  of  penetration 


■mi 


116 


4      Joining  of  Metals 


FIGURE    4.40 

Undercutting 


FIGURE    4.41 

Underfilling 


Underfilling.  Underfilling,  shown  in  Figure  4.41,  involves  a  depression  in  the  weld 
face  below  the  surface  of  the  adjoining  base  metal.  More  filler  metal  has  to  be  added 
in  order  to  prevent  this  defect. 


Testing  and  Inspection  of  Welds 

Welds  must  be  evaluated  by  being  subjected  to  testing  according  to  codes  and  specifi- 
cations that  are  different  for  different  countries.  The  various  types  of  tests  can  be  di- 
vided into  two  groups:  destructive  and  nondestructive.  Destructive  testing  always 
results  in  destroying  the  specimen  (the  welded  joint)  and  rendering  it  unsuitable  for  its 
design  function.  Destructive  tests  can  be  mechanical,  metallurgical,  or  chemical.  We 
next  review  various  destructive  and  nondestructive  testing  methods. 

Visual  inspection.  Visual  inspection  involves  examination  of  the  weld  by  the  naked 
eye  and  checking  its  dimensions  by  employing  special  gages.  Defects  such  as  cracks, 
porosity,  undercuts,  underfills,  or  overlaps  can  be  revealed  by  this  technique. 

Mechanical  tests.  Mechanical  tests  are  generally  similar  to  the  conventional  me- 
chanical tests,  the  difference  being  the  shape  and  size  of  the  test  specimen.  Tensile, 
bending,  impact,  and  hardness  tests  are  carried  out.  Such  tests  are  conducted  either  on 
the  whole  welded  joint  or  on  the  deposited  metal  only. 

Metallurgical  tests.  Metallurgical  tests  involve  metallurgical  microstructure  and 
macrostructure  examination  of  specimens.  Macrostructure  examination  reveals  the 
depth  of  penetration,  the  extent  of  the  heat-affected  zone,  and  the  weld  bead  shape,  as 
well  as  hidden  cracks,  porosity,  and  slag  inclusions.  Microstructure  examination  can 
show  the  presence  of  nitrides,  martensite,  or  other  structures  that  cause  metallurgically 
oriented  welding  problems. 

Chemical  tests.  Chemical  tests  are  carried  out  to  ensure  that  the  composition  of  the 
filler  metal  is  identical  to  that  specified  by  the  manufacturing  engineer.  Some  are  crude 
tests,  such  as  spark  analysis  or  reagent  analysis;  however,  if  accurate  data  are  required, 
chemical  analysis  or  spectrographic  testing  must  be  carried  out. 

Radiographic  inspection.  Radiographic  inspection  is  usually  performed  by  employ- 
ing industrial  X  rays.  This  technique  can  reveal  hidden  porosity,  cracks,  and  slag  in- 
clusions. It  is  a  nondestructive  test  that  does  not  destroy  the  welded  joint. 
High-penetration  X  rays  are  sometimes  also  employed  for  inspecting  weldments  hav- 
ing thicknesses  up  to  l!/2  inches  (37  mm). 


4.2  Welding 


117 


Pressure  test.  Hydraulic  (or  air)  pressure  is  applied  to  welded  conduits  that  are 
going  to  be  subjected  to  pressure  during  their  service  lives  to  check  their  tightness  and 
durability. 

Ultrasonic  testing.  Ultrasonic  waves  with  frequencies  over  20  kHz  are  employed  to 
detect  various  kinds  of  flaws  in  the  weld,  such  as  the  presence  of  nonmetallic  inclu- 
sions, porosity,  and  voids.  This  method  is  reliable  even  for  testing  very  thick  parts. 

Magnetic  testing.  As  we  know  from  physics,  the  lines  of  magnetic  flux  are  distorted 
in  such  a  way  as  to  be  concentrated  at  the  sides  of  a  flaw  or  a  discontinuity,  as  seen  in 
Figure  4.42a  and  b.  This  test,  therefore,  involves  magnetizing  the  part  and  then  using 
fine  iron-powder  particles  that  were  uniformly  dispersed  on  the  surface  of  the  part  to 
reveal  the  concentration  of  the  flux  lines  at  the  location  of  the  flaw.  This  method  is  suc- 
cessful in  detecting  superficial  hair  cracks  and  pores  in  ferrous  metal. 

Ammonia  penetrant  test.  The  ammonia  penetrant  test  is  used  to  detect  any  leakage 
from  welded  vessels.  It  involves  filling  the  vessel  with  a  mixture  of  compressed  air  and 
ammonia  and  then  wrapping  it  with  paper  that  has  been  impregnated  in  a  5  percent  so- 
lution of  mercuric  nitrate.  Any  formation  of  black  spots  is  an  indication  of  leakage. 

Fluorescent  penetrant  test.  The  part  is  immersed  for  about  half  an  hour  in  oil  (or 
an  oil  mixture)  and  then  dipped  in  magnesia  powder.  The  powder  adheres  at  any 
crack  location. 

Design  Considerations 

As  soon  as  the  decision  is  made  to  fabricate  a  product  by  welding,  the  next  step  is  to 
decide  which  welding  process  to  use.  This  decision  should  be  followed  by  selection  of 
the  types  of  joints,  by  determination  of  the  locations  and  distribution  of  the  welds,  and, 
finally,  by  making  the  design  of  each  joint.  Following  is  a  brief  discussion  of  the  fac- 
tors to  be  considered  in  each  design  stage. 

Selection  of  the  joint  type.  We  have  previously  discussed  the  various  joint  designs 
and  realized  that  the  type  of  joint  depends  upon  the  thickness  of  the  parts  to  be  welded. 
In  fact,  there  are  other  factors  that  should  also  affect  the  process  of  selecting  a  partic- 
ular type  of  joint.  For  instance,  the  magnitude  of  the  load  to  which  the  joint  is  going 


FIGURE    4.42 

Magnetic  testing  of 
welds:  (a)  defective 
weld;  (b)  sound  weld 


(a) 


(b) 


118 


Joining  of  Metals 


to  be  subjected  during  its  service  life  is  one  other  important  factor.  The  manner  in 
which  the  load  is  applied  (i.e.,  impact,  steady,  or  fluctuating)  is  another  factor.  Whereas 
the  square  butt,  simple- V,  double-V,  and  simple-U  butt  joints  are  suitable  only  for 
usual  loading  conditions,  the  double-U  butt  joint  is  recommended  for  all  loading  con- 
ditions. On  the  other  hand,  the  square-T  joint  is  appropriate  for  carrying  longitudinal 
shear  under  steady-state  conditions.  When  severe  longitudinal  or  transverse  loads  are 
anticipated,  other  types  of  joints  (e.g.,  the  single-bevel-T,  the  double-bevel-T,  and  the 
double-J)  have  to  be  considered.  In  all  cases,  it  is  obvious  that  cost  is  the  decisive  fac- 
tor whenever  there  is  a  choice  between  two  types  of  joints  that  would  function  equally 
well. 

Location  and  distribution  of  welds.  It  has  been  found  that  the  direction  of  the  linear 
dimension  of  the  weld  with  respect  to  the  direction  of  the  applied  load  has  an  effect  on 
the  strength  of  the  weld.  In  fact,  it  has  been  theoretically  and  experimentally  proven 
that  a  lap  weld  whose  linear  direction  is  normal  to  the  direction  of  the  applied  load,  as 
is  shown  in  Figure  4.43a,  is  30  percent  stronger  than  a  lap  weld  whose  linear  direction 
is  parallel  to  the  direction  of  the  applied  load,  as  shown  in  Figure  4.43b.  In  the  first 
case,  the  maximum  force  F  that  the  joint  can  carry  without  any  signs  of  failure  can  be 
approximated  by  the  following  equation: 

F  =  0.707  Xhfx  Gallowable  (4.3) 

where:  £  is  the  weld  leg 

W  is  the  length  of  the  weld 

^allowable  is  the  allowable  tensile  stress  of  the  filler  material  (e.g., 
electrode) 

In  the  second  case  (Figure  4.43b),  the  strength  of  the  joint  is  based  on  the  fact  that  the 
throat  plane  of  the  weld  is  subjected  to  pure  shear  stress  and  is  given  by  the  following 
equation: 


F  =  0.707  x  €  x  W  x  xallowable 

where:  €  is  the  weld  leg 

W  is  the  length  of  the  weld 

^allowable  's  the  allowable  shear  stress  of  the  electrode 


(4.4) 


FIGURE    4.43 

Location  and 
distribution  of  welds: 
(a)  weld  linear  direction 
normal  to  the  applied 
load;  (b)  weld  linear 
direction  parallel  to  the 
applied  load 


Force 


Force 


(a) 


(b) 


4.2  Welding 


119 


From  the  theory  of  plasticity,  assuming  you  adopt  the  same  safety  factor  in  both  cases, 
it  is  easy  to  prove  that 


1 

^allowable  —         /       ^allowable 


=  0.565  G:, 


(4.5) 


On  the  other  hand,  the  strength  of  a  butt-welded  joint  can  be  given  by  the  following 
equation: 


F  =  €  x  W  x  aa 


(4.6) 


where  t,  W,  and  aallowable  are  as  previously  mentioned.  A  product  designer  should, 
therefore,  make  use  of  this  characteristic  when  planning  the  location  and  distribution 
of  welds. 

Another  important  point  to  consider  is  the  prevention  of  any  tendency  of  the 
welded  elements  to  rotate  when  subjected  to  mechanical  loads.  A  complete  force 
analysis  must  be  carried  out  in  order  to  determine  the  proper  length  of  each  weld.  Let 
us  now  consider  a  practical  example  to  see  the  cause  and  the  remedy  for  this  tendency 
to  rotate.  Figure  4.44  shows  an  L  angle  welded  to  a  plate.  Any  load  applied  through  the 
angle  will  pass  through  its  center  of  gravity.  Therefore,  the  resisting  forces  that  act 
through  the  welds  will  not  be  equal;  the  force  closer  to  the  center  of  gravity  of  the 
angle  will  always  be  larger.  Consequently,  if  any  tendency  to  rotate  is  to  be  prevented, 
the  weld  that  is  closer  to  the  center  of  gravity  must  be  longer  than  the  other  one.  Using 
simple  statics,  it  can  easily  be  seen  that 


W2 


(4.7) 


It  is  also  recommended  that  very  long  welds  be  avoided.  It  has  been  found  that  two 
small  welds,  for  example,  are  much  more  effective  than  a  single  long  weld. 

Joint  design.     In  addition  to  the  procedures  and  rules  adopted  in  common  design  prac- 
tice, there  are  some  guidelines  that  apply  to  joint  design: 


FIGURE    4.44 

Preventing  the  tendency 
of  the  welded  element 
to  rotate  by  appropriate 
distribution  of  welds 


y- 


w. 


xxxx 


■><■ 7' 


w 


Angle 


Center  of 
gravity 


Center  of  gravity 
-of  the  angle 


120 


4      Joining  of  Metals 


FIGURE    4.45 

Designs  that  promote 
or  eliminate  distortion 
in  welding: 

(a)  distortion  caused  by 
unbalanced  weld; 

(b)  and  (c)  methods  for 
reducing  distortion 


(a) 


(b) 


(0 


1.  Try  to  ensure  accessibility  to  the  locations  where  welds  are  to  be  applied. 

2.  Try  to  avoid  overhead  welding. 

3.  Consider  the  heating  effect  on  the  base  metal  during  the  welding  operation.  Balance 
the  welds  to  minimize  distortion.  Use  short,  intermittent  welds.  Figure  4.45a  shows 
distortion  caused  by  an  unbalanced  weld,  whereas  Figure  4.45b  and  c  shows  meth- 
ods for  reducing  that  distortion. 

4.  Avoid  crevices  around  welds  in  tanks  as  well  as  grooves  (and  the  like)  that  would 
allow  dirt  to  accumulate.  Failure  to  do  so  may  result  in  corrosion  in  the  welded 
joint. 

5.  Do  not  employ  welding  to  join  steels  with  high  hardenability. 

6.  Do  not  weld  low-carbon  steels  to  alloy  steels  by  the  conventional  fusion  welding 
methods  because  they  have  different  critical  cooling  rates  and  hence  cannot  be  suc- 
cessfully welded. 

7.  When  employing  automatic  welding  (e.g.,  submerged  arc),  the  conventional  joint 
design  of  manual  welding  should  be  changed.  Wider  Vs  (for  butt  joints)  are  used, 
and  single-pass  welds  replace  multipass  welds. 


4.3  SURFACING  AND  HARD-FACING 

Surfacing  involves  the  application  of  a  thin  deposit  on  the  surface  of  a  metallic  work- 
piece  by  employing  a  welding  method  such  as  oxyacetylene-gas  welding,  shielded- 
metal  arc  welding,  or  automatic  arc  welding.  The  process  is  carried  out  to  increase  the 
strength,  the  hardness,  and  the  resistance  to  corrosion,  abrasion,  or  wear.  For  the  last 
reason,  the  process  is  commonly  known  as  hard-facing. 

Good  hard-facing  practice  should  be  aimed  at  achieving  a  strong  bond  between 
the  deposit  and  the  base  metal  and  also  at  preventing  the  formation  of  cracks  and  other 
defects  in  the  deposited  layer.  Therefore,  the  deposited  layer  should  not  generally  ex- 
ceed 3/32  inch  (2  mm)  and  will  rarely  exceed  1/4  inch  (6  mm).  Also,  the  base  metal 


4.4  Thermal  Cutting  of  Metals  121 

should  be  heated  to  a  temperature  of  500°F  to  950°F  (350°C  to  500°C)  to  ensure  a 
good  metallurgical  bond  and  to  allow  the  deposited  layer  to  cool  down  slowly. 

Hard-facing  permits  the  use  of  very  hard  wear-  and  corrosion-resisting  com- 
pounds. The  materials  used  in  this  process  are  complex.  They  involve  hard  com- 
pounds, like  carbides  and  borides,  that  serve  as  the  wear-resisting  elements,  and  a 
tough  matrix  composed  of  air-hardening  steel  or  iron-base  alloys.  Such  deposited  ma- 
terials increase  the  service  life  of  a  part  three-  or  fourfold.  The  process  is  also  em- 
ployed in  restoring  worn  parts. 

The  process  of  hard-facing  has  found  widespread  application  in  the  heavy  con- 
struction equipment  industry,  in  mining,  in  agricultural  machinery,  and  in  the  petro- 
leum industry.  The  list  of  parts  that  are  usually  hard-faced  is  long  and  includes,  for 
example,  the  vulnerable  surfaces  of  chemical-process  vessels,  pump  liners,  valve  seats, 
drive  sprockets,  ripper  teeth,  shovel  teeth,  chutes,  and  the  edges  of  coal  recovery 
augers. 


4.4  THERMAL  CUTTING  OF  METALS 


In  this  section,  we  discuss  the  thermal  cutting  of  metals,  specifically  oxyfuel  flame  cut- 
ting and  the  different  arc  cutting  processes.  Although  all  these  processes  do  not,  by  any 
means,  fall  under  the  topic  of  joining  (the  action  involved  is  opposite  to  that  of  join- 
ing), they  employ  the  same  equipment  as  the  corresponding  welding  process  in  each 
case.  The  thermal  cutting  processes  are  not  alternatives  to  sawing  but  rather  are  used 
for  cutting  thick  plates,  1  to  10  inches  (25  to  250  mm)  thick,  as  well  as  for  difficult-to- 
machine  materials.  Thermal  cutting  may  be  manual,  using  a  hand-operated  cutting 
torch  (or  electrode),  or  the  cutting  element  can  be  machine  driven  by  a  numerically 
controlled  system  or  by  special  machines  called  radiographs. 

Oxyfuel  Cutting 

Oxyfuel  cutting  (OFC)  is  similar  to  oxyfuel  welding  except  that  an  oxidizing  flame 
must  always  be  used.  The  process  is  extensively  used  with  ferrous  metal  having  thick- 
nesses up  to  10  inches  (250  mm).  During  the  process,  red-hot  iron,  directly  subjected 
to  the  flame,  is  oxidized  by  the  extra  oxygen  in  the  flame;  it  then  burns  up,  leaving  just 
ashes  or  slag.  Also,  the  stream  of  burning  gases  washes  away  any  molten  metal  in  the 
region  being  cut.  Generally,  there  is  a  relationship  between  the  speed  of  travel  of  the 
torch  or  electrode  and  the  smoothness  of  the  cut  edge:  The  higher  the  speed  of  travel, 
the  coarser  the  cut  edge. 

Although  acetylene  is  commonly  used  as  a  fuel  in  this  process,  other  gases  are 
also  employed,  including  butane,  methane,  propane,  natural  gas,  and  a  newly  devel- 
oped gas  with  the  commercial  name  Mapp.  Hydrogen  is  sometimes  used  as  a  fuel,  es- 
pecially underwater  to  provide  a  powerful  preheating  flame.  In  this  case,  compressed 
air  is  used  to  keep  water  away  from  the  flame. 

The  oxyfuel  cutting  process  can  be  successfully  employed  only  when  the  ignition 
temperature  of  the  metal  being  cut  is  lower  than  its  melting  point.  Another  condition  for 
the  successful  application  of  the  process  involves  ensuring  that  the  melting  points  of  the 


122  4     Joining  of  Metals 


formed  oxides  are  lower  than  that  of  the  base  metal  itself.  Therefore,  oxyfuel  cutting  is 
not  recommended  for  cast  iron  because  its  ignition  temperature  is  higher  than  its  melt- 
ing point.  The  process  is  also  not  appropriate  for  cutting  stainless  steel,  high-alloy 
chromium  and  chrome-nickel  alloys,  and  nonferrous  alloys  because  the  melting  points 
of  the  oxides  of  these  metals  are  higher  than  the  melting  points  of  the  metals  themselves. 

Arc  Cutting 

There  are  several  processes  based  upon  utilization  of  the  heat  generated  by  an  electric 
arc.  These  arc  cutting  processes  are  generally  employed  for  cutting  nonferrous  metals, 
medium-carbon  steel,  and  stainless  steel. 

Conventional  arc  cutting.  Conventional  arc  cutting  is  similar  to  shielded-metal  arc 
welding.  It  should  always  be  remembered,  however,  that  the  electrode  enters  the  gap 
of  the  cut,  so  the  coating  must  serve  as  an  insulator  to  keep  the  electric  arc  from  short- 
ing out.  Consequently,  electrodes  with  coatings  containing  iron  powder  are  not  recom- 
mended for  use  with  this  process. 

Air  arc  cutting.  Air  arc  cutting  involves  preheating  the  metal  to  be  cut  by  an  electric 
arc  and  blowing  out  the  resulting  molten  metal  by  a  stream  of  compressed  air.  The  arc- 
air  torch  is  actually  a  steel  tube  through  which  compressed  air  is  blown. 

Oxygen  arc  cutting.  Oxygen  arc  cutting  is  similar  to  air  arc  cutting  except  that  oxy- 
gen is  blown  instead  of  air.  The  process  is  capable  of  cutting  cast  irons  and  stainless 
steels  with  thicknesses  up  to  2  inches  (50  mm). 

Carbon  arc  cutting.  In  carbon  arc  cutting,  a  carbon  or  graphite  electrode  is  used.  The 
process  has  the  disadvantage  of  consuming  that  electrode  quickly,  especially  if  contin- 
uous cutting  is  carried  out. 

Tungsten  arc  cutting.  The  electrode  used  in  tungsten  arc  cutting  is  made  of  tungsten 
and  has,  therefore,  a  service  life  that  is  far  longer  than  that  of  the  carbon  or  graphite 
electrodes.  Tungsten  arc  cutting  is  commonly  employed  for  stainless  steel,  copper, 
magnesium,  and  aluminum. 

Air-carbon  arc  cutting.  Air-carbon  arc  cutting  is  quite  similar  to  carbon  arc  cutting, 
the  difference  being  the  use  of  a  stream  of  compressed  air  to  blow  the  molten  metal 
(that  has  been  fused  by  the  arc)  out  of  the  kerf  (groove).  The  process  cuts  almost  all 
metals  because  its  mechanics  involve  oxidation  of  the  metal.  Its  applications  involve 
removal  of  welds,  removal  of  defective  welds,  and  dismantling  of  steel  structures. 

Plasma  arc  cutting.  A  plasma  arc  is  employed  to  cut  metals  in  plasma  arc  cutting 
(PAC).  The  temperature  of  the  plasma  jet  is  extremely  high  (ten  times  higher  than  that 
obtained  with  oxyfuel),  thus  enabling  high-speed  cutting  rates  to  be  achieved.  Also,  as 
a  consequence,  the  heat-affected  zone  formed  along  the  edge  of  the  kerf  is  usually  less 
than  0.05  inch  (1.3  mm).  Plasma  arc  cutting  can  be  used  for  cutting  stainless  steel  as 
well  as  hard-to-cut  alloys.  A  modification  of  the  process  involves  using  a  special  noz- 
zle to  generate  a  whirlpool  of  water  on  the  workpiece,  thus  increasing  the  limit  on  the 


4.5  Brazing  and  Soldering 


123 


FIGURE    4.46 

Laser-beam  cutting  of 
sheets  and  plates 


Focusing 
lens 


Material— ^^^     ^^^ 


Molten  material 


thickness  of  the  workpiece  up  to  3  inches  (75  mm)  and  meanwhile  improving  the  qual- 
ity of  the  cut.  The  only  limitation  on  plasma  arc  cutting  is  that  the  workpiece  must  be 
electrically  conductive. 

Laser-beam  cutting.  The  basic  principles  of  laser-beam  cutting  are  similar  to  those  of 
laser-beam  welding.  Nevertheless,  laser  cutting  is  achieved  by  the  pressure  from  a  jet 
of  gas  that  is  coaxial  with  the  laser  beam,  as  shown  in  Figure  4.46.  The  function  of  the 
gas  jet  is  to  blow  away  the  molten  metal  that  has  been  fused  by  the  laser  beam.  Laser 
beams  can  be  employed  in  cutting  almost  any  material,  including  nonconductive  poly- 
mers and  ceramics.  Also,  the  process  is  usually  automated  by  using  computerized  nu- 
merical control  systems  to  control  the  movements  of  the  machine  table  under  the  laser 
beam  so  that  workpieces  can  be  cut  to  any  desired  contour.  Other  advantages  of  the 
laser-beam  cutting  process  include  the  straight-edged  kerfs  obtained,  the  very  narrow 
heat-affected  zone  that  results,  and  the  elimination  of  the  part  distortion  experienced 
with  other  conventional  thermal  cutting  processes. 


RAZING  AND  SOLDERING 


Brazing  and  soldering  are  processes  employed  for  joining  solid  metal  components  by 
heating  them  to  the  proper  temperature  and  then  introducing  between  them  a  molten 
filler  alloy  (brazing  metal  or  solder).  The  filler  alloy  must  always  have  a  melting  point 
lower  than  that  of  the  base  metal  of  the  components.  The  filler  alloy  must  also  possess 
high  fluidity  and  wettability  (i.e.,  be  able  to  spread  and  adhere  to  the  surface  of  the 
base  metal).  As  you  may  expect,  the  mechanics  of  brazing  or  soldering  are  different 
from  those  of  welding.  A  strong  brazed  joint  is  obtained  only  if  the  brazing  metal  can 
diffuse  into  the  base  metal  and  form  a  solid  solution  with  it.  Figure  4.47a  and  b  is  a 
sketch  of  the  microstructure  of  two  brazed  joints  and  is  aimed  at  clarifying  the  me- 
chanics of  brazing  and  soldering. 

Brazing  and  soldering  can  be  employed  to  join  carbon  and  alloy  steels,  nonfer- 
rous  alloys,  and  dissimilar  metals.  The  parts  to  be  joined  together  must  be  carefully 
cleaned,  degreased,  and  clamped.  Appropriate  flux  is  applied  to  remove  any  re- 
maining oxide  and  to  prevent  any  further  oxidation  of  the  metals.  It  is  only  under 


124 


4      Joining  of  Metals 


FIGURE    4.47 

A  sketch  of  the 
micro-structure  of  two 
brazed  joints:  (a)  gold 
base  metal;  (b)  low- 
carbon  steel  base 
metal 


Au-Cu 
solder 


Diffusion 
along  grain 
boundaries 


(a) 


(b) 


such  conditions  that  the  filler  metal  can  form  a  strong  metallic  bond  with  the  base 
metal. 

The  main  difference  between  soldering  and  brazing  is  the  melting  point  of  the 
filler  metal  in  each  case.  Soft  solders  used  in  soldering  have  melting  points  below 
930°F  (500°C)  and  produce  joints  with  relatively  low  mechanical  strength,  whereas 
hard  solders  (brazing  metals)  have  higher  melting  points,  up  to  1650°F  (900°C),  and 
produce  joints  with  high  mechanical  strength. 

Soft  solders  are  low-melting-point  eutectic  alloys.  They  are  basically  tin,  lead, 
cadmium,  bismuth,  or  zinc  alloys.  On  the  other  hand,  brazing  filler  metals  are  alloys 
consisting  mainly  of  copper,  silver,  aluminum,  magnesium,  or  nickel.  Table  4.2  gives 
the  recommended  filler  alloys  for  different  base  metals.  The  chemical  composition 
and  the  field  of  application  for  the  commonly  used  soft  and  hard  solders  are  given  in 
Table  4.3. 


TABLE    4.2 

Recommended  filler  alloys  for  different  base  metals 


Filler  Alloys 

Silver 

40%  Zinc 

Nickel 

Base  Metal 

Tin-Lead 

Solder 

Brass 

Silver                  Copper 

Steel 

* 

* 

X 

*                                         X 

Stainless  steel 

* 

X 

* 

X 

Nickel  alloys 

* 

* 

X 

X 

Copper  (pure) 

* 

X 

X 

Brass  or  bronze 

* 

X 

Silver  (pure) 

* 

X 

Not  allowed 

Aluminum 

Either  Sn-Zn  10 

Al-Mg  3,  Al-Si  5 

or  Al-Si  12 

*  =  may  be  used 
x  =  best  used 


4.5  Brazing  and  Soldering 


125 


TABLE    4.3 

Most  commonly  used  soft  and  hard  brazing  filler  metals 


f 

Approximate  Chemical 

Brazing  or  Soldering 

^ 

Type 

Composition 

Temperature 

Application 

Tin  solder 

Sn-Pb  70 

360-490°F  (183-255°C) 

General-purpose  solder 

Sn-Pb  50 

360-420°F (183-216°C) 

Electrical  application 

Sn-Zn  10 

509-750°F  (265-400°C) 

Soft  soldering  of 
aluminum 

Silver  solder 

Ag  25-50,  Cu  20-40,  Sn  0-35, 

1175-1550°F  (635-845°C) 

Brazing  of  copper  alloys 

Cd  0-20,  Zn  0-20 

and  silver  in  electronics 

Brass  solder 

Cu-Zn  40 

1670-1750°F  (910-955°C) 

General-purpose  hard 
solder 

Nickel  silver 

Cu  balance,  Zn  20-30, 

1720-1800° F (938-982°C) 

Nickel  alloys  and  steel 

Ni  10-20 

brazing 

Copper 

99.9%  copper 

2000-2100°F  (1093-1150°C) 

Brazing  of  steel 

Silumin 

Al-Si  12 

1080-1120°F (582-605°C) 

Brazing  all  aluminum 
alloys  except  silumin 

Fluxes 

Fluxes  are  employed  in  soft  soldering  as  well  as  in  brazing  in  order  to  protect  the 
cleaned  surfaces  of  the  base  metal  against  oxidation  during  those  processes.  In  addi- 
tion, fluxes  enable  proper  wetting  of  the  surfaces  of  the  base  metals  by  the  molten  filler 
solders. 

There  are  two  kinds  of  fluxes  for  soft  soldering  operations:  organic  and  inor- 
ganic. The  inorganic  fluxes  are  mostly  aqueous  solutions  of  zinc  and/or  ammonium 
chlorides.  They  must,  however,  be  completely  removed  after  the  soldering  operation 
because  of  their  corrosive  effect.  It  is,  therefore,  completely  forbidden  to  use  inor- 
ganic fluxes  in  soldering  electronic  components.  On  the  other  hand,  organic  fluxes  do 
not  have  such  corrosive  effects  and  are,  therefore,  widely  used  for  fine  soldering  in 
electronic  circuits.  The  commonly  used  organic  fluxes  involve  colophony,  a  kind  of 
resin  with  a  melting  point  between  350°F  and  390°F  (180°C  and  200°C),  as  well  as 
some  fats. 

The  fluxes  employed  in  brazing  include  combinations  of  borox,  boric  acid,  bo- 
rates, fluorides,  and  fluoborates  together  with  a  wetting  agent.  The  flux  can  be  in  the 
form  of  a  liquid,  slurry,  powder,  or  paste,  depending  upon  the  brazing  method  used. 

Soldering  Techniques 

The  manual  soldering  method  involves  using  a  hand-type  soldering  iron  that  is  made 
of  copper  and  has  to  be  tinned  each  time  before  use.  The  iron  is  first  heated  to  a  tem- 
perature of  about  570°F  (300°C),  and  its  tip  is  then  dipped  into  the  flux  and  tinned  with 


126  4     Joining  of  Metals 


the  solder.  Next,  the  iron  is  used  for  heating  the  prepared  surfaces  of  the  base  metal 
and  for  melting  and  distributing  the  soft  solder.  When  the  solder  solidifies,  it  forms  the 
required  solder  seam. 

Several  other  methods  are  also  used  for  soldering.  These  include  dip  soldering  and 
induction  soldering  as  well  as  the  use  of  guns  (blowtorches).  Nevertheless,  electric  sol- 
dering irons  are  still  quite  common. 

Brazing  Techniques 

The  selection  of  a  preferred  brazing  method  has  to  be  based  on  the  size  and  shape  of 
the  joined  components,  the  base  metal  of  the  joint,  the  brazing  filler  metal  to  be  used, 
and  the  production  rate.  When  two  brazing  techniques  are  found  to  be  equally  suit- 
able, cost  is  the  deciding  factor.  The  following  brazing  methods  are  commonly  used 
in  industry. 

Torch  brazing.  Torch  brazing  is  still  the  most  commonly  used  method.  It  is  very 
similar  to  oxyfuel  flame  welding  in  that  the  source  of  heat  is  a  flame  obtained  from 
the  combustion  of  a  mixture  of  a  fuel  gas  (e.g.,  acetylene)  and  oxygen.  The  process 
is  very  popular  for  repair  work  on  cast  iron  and  is  usually  applied  manually,  al- 
though it  can  be  used  on  a  semiautomatic  basis.  In  this  process,  however,  a  reduc- 
ing flame  should  be  used  to  heat  the  joint  area  to  the  appropriate  brazing 
temperature.  A  flux  is  then  applied,  and  as  soon  as  it  melts,  the  filler  metal  (braz- 
ing alloy)  is  hand-fed  to  the  joint  area.  When  the  filler  metal  melts,  it  flows  into  the 
clearance  between  the  base  components  by  capillary  attraction.  The  filler  metal 
should  always  be  melted  by  the  heat  gained  by  the  joint  and  not  by  directly  apply- 
ing the  flame. 

Furnace  brazing.  Furnace  brazing  is  performed  in  either  a  batch  or  a  continuous  con- 
veyor-type furnace  and  is,  therefore,  best  suited  for  mass  production.  The  atmosphere 
of  the  furnace  is  controlled  to  prevent  oxidation  and  to  suit  the  metals  involved  in  the 
process.  That  atmosphere  can  be  dry  hydrogen,  dissociated  ammonia,  nitrogen,  argon, 
or  any  other  inert  gas.  Vacuum  furnaces  are  also  employed,  especially  with  brazing 
materials  containing  titanium  or  aluminum.  Nevertheless,  a  suitable  flux  is  often  em- 
ployed. The  filler  metal  must  be  placed  in  the  joint  before  the  parts  go  inside  the  fur- 
nace. The  filler  metal  can,  in  this  case,  take  the  form  of  a  ring,  washer,  wire,  powder, 
or  paste. 

Induction  brazing.  In  induction  brazing,  the  components  to  be  brazed  are  heated  by 
placing  them  in  an  alternating  magnetic  field  that,  in  turn,  induces  an  alternating  cur- 
rent in  the  components  that  rapidly  reverses  its  direction.  Special  coils  made  of  copper, 
referred  to  as  inductors,  are  employed  for  generating  the  magnetic  field.  The  filler 
metal  is  often  placed  in  the  joint  area  before  brazing  but  can  also  be  hand-fed  by  the 
operator.  This  technique  has  a  clear  advantage,  which  is  the  possibility  of  obtaining  a 
very  closely  controlled  heating  area. 

Dip  brazing.  Dip  brazing  involves  dipping  the  joint  to  be  brazed  in  a  molten  filler 
metal.  The  latter  is  maintained  in  a  special  externally  heated  crucible  and  is  covered 


4.5  Brazing  and  Soldering  127 

with  a  layer  of  flux  to  protect  it  from  oxidation.  Because  the  filler  metal  coats  the  en- 
tire workpiece,  this  process  is  used  only  for  small  parts. 

Salt-bath  brazing.  The  source  of  heating  in  salt-bath  brazing  is  a  molten  bath  of  fluo- 
ride and  chloride  salts.  The  filler  metal  is  placed  in  the  joint  area  before  brazing  and  is 
also  sometimes  cladded.  Next,  the  whole  assembly  is  preheated  to  an  appropriate  tem- 
perature and  then  dipped  for  1  to  6  minutes  in  the  salt  bath.  Finally,  the  hot  brazed  joint 
is  rinsed  thoroughly  in  hot  and  cold  water  to  remove  any  remaining  flux  or  salt.  Gen- 
erally, this  process  is  employed  for  brazing  aluminum  and  its  alloys.  There  is,  however, 
a  problem  associated  with  the  process,  and  that  is  the  pollution  caused  by  the  effluent 
resulting  from  the  rinsing  operation. 

Resistance  brazing.  Low-voltage,  high-amperage  current  is  used  as  the  source  of 
energy  in  resistance  brazing,  as  is  the  case  with  spot  welding.  In  fact,  a  spot  welder 
can  be  employed  to  carry  out  this  process,  provided  that  the  pressure  is  carefully  ad- 
justed so  as  to  be  just  enough  to  secure  the  position  of  the  contact  where  heat  devel- 
ops. The  workpiece  is  held  between  the  two  electrodes,  with  the  filler  metal 
preloaded  at  the  joint  area.  This  process  is  normally  used  for  brazing  of  electrical 
contacts  and  in  the  manufacture  of  copper  transformer  leads. 

Design  of  Brazed  Joints 

For  the  proper  design  of  brazed  joints,  two  main  factors  have  to  be  taken  into  consid- 
eration. The  first  factor  involves  the  mechanics  of  the  process  in  that  the  brazing  filler 
metal  flows  through  the  joint  by  capillary  attraction.  The  second  factor  is  that  the 
strength  of  the  filler  metal  is  poorer  than  that  of  the  base  metals.  The  product  designer 
should  aim  for  the  following: 

1.  Ensuring  that  the  filler  metal  is  placed  on  one  side  of  the  joint  and  allocating  a 
space  for  locating  the  filler  metal  before  (or  during)  the  process. 

2.  Adjusting  the  joint  clearance  in  order  to  ensure  optimum  conditions  during  brazing. 
That  clearance  is  dependent  upon  the  filler  metal  used  and  normally  takes  a  value 
less  than  0.005  inch  (0.125  mm),  except  for  silumin,  in  which  case  it  can  go  up  to 
0.025  inch  (0.625  mm). 

3.  Ensuring  that  the  distance  to  be  traveled  by  the  filler  metal  is  shorter  than  the  limit 
distant,  as  dictated  by  the  physics  of  capillarity. 

4.  Providing  enough  filler  metal. 

5.  Increasing  the  area  of  the  joint  because  the  filler  metal  is  weaker  than  the  base  metal. 

There  are  three  types  of  joint-area  geometries:  butt,  scarf,  and  lap.  The  butt  joint 
is  the  weakest,  and  the  lap  is  the  strongest.  Nevertheless,  when  designing  lap  joints, 
make  sure  that  the  joint  overlap  is  more  than  3f,  where  t  is  the  thickness  of  the  thinner 
parent  metal.  Examples  of  some  good  and  poor  practices  in  the  design  of  brazed  joints 
are  shown  in  Figure  4.48  as  guidelines  for  beginners  in  product  design.  Also,  always 
remember  that  brazed  joints  are  designed  to  carry  shear  stress  and  not  tension. 


128 


4      Joining  of  Metals 


FIGURE    4.48 

Good  and  poor 
practices  in  the  design 
of  brazed  joints 


essssxzzzzzi 

Poor 


sssss^zzzza 

Good 


ESSSZZZZZ 
Better 


<  3t 

ggF23 


Best 


Best 


w 


Poor 


77t\ 


w* 


Good 


Poor 


Good 


Jzzzzzzi 


m 


Poor 


S KS/////A 

X 


Good 


(SSS3E2ZZZ3 


tsssizzzzzj   IzzzzS 


tzzzz 


SI 


•^vq 


SSSSS3 


Poor 


Good 


TICKING  OF  METALS 


Sticking,  or  adhesive  bonding,  of  metals  is  becoming  very  popular  in  the  automotive, 
aircraft,  and  packaging  industries  because  of  the  advantages  that  this  technique  can 
offer.  Thanks  to  the  recent  development  in  the  chemistry  of  polymers,  adhesives  are 
now  cheap,  can  be  applied  easily  and  quickly,  and  can  produce  reasonably  strong 
joints.  Adhesive  bonding  can  also  be  employed  in  producing  joints  of  dissimilar  met- 
als or  combinations  of  metals  and  nonmetals  like  ceramics  or  polymers.  This  certainly 
provides  greater  flexibility  when  designing  products  and  eliminates  the  need  for  com- 
plicated, expensive  joining  processes. 

As  we  know,  it  is  possible  to  stick  entirely  smooth  metal  surfaces  together.  It  is 
obvious,  therefore,  that  the  sticking  action  is  caused  by  adhesive  forces  between  the 
sticking  agent  and  the  workpiece  and  not  by  the  flowing  and  solidification  of  the  stick- 
ing agent  into  the  pores  of  the  workpiece  as  occurs,  for  instance,  with  wood.  In  other 
words,  adhesion  represents  attractive  intermolecular  forces  under  whose  influence  the 
particles  of  a  surface  adhere  to  those  of  another  one.  There  are  also  many  opinions  sup- 
porting the  theory  that  mechanical  interlocking  plays  a  role  in  bonding. 

Adhesives 

Structural  adhesives  are  normally  systems  including  one  or  more  polymeric  materials. 
In  their  unhardened  state  (i.e.,  before  they  are  applied  and  cured),  these  adhesives 
can  take  the  form  of  viscous  liquids  or  solids  with  softening  temperatures  of  about 
212°F  (100°C).  The  unhardened  adhesive  agents  are  often  soluble  in  ketones,  esters, 
and  higher  alcohols,  as  well  as  in  aromatic  and  chlorine  hydrocarbons.  The  hardened 


4.6  Sticking  of  Metals 


129 


FIGURE    4.49 

The  three  types  of 
adhesive-bonded  joints 


adhesives,  however,  resist  nearly  all  solvents.  Adhesives  that  find  industrial  application 
in  bonding  two  nonmetallic  workpieces  include  cyanacrylates,  acrylics,  and  poly- 
urethanes.  Following  is  a  brief  description  of  the  adhesives  that  are  commonly  used  in 
industry. 

Epoxies.  Epoxies  are  thermosetting  polymers  (see  Chapter  8)  that  require  the  addi- 
tion of  a  hardener  or  the  application  of  heat  so  that  they  can  be  cured.  Epoxies  are  con- 
sidered to  be  the  best  sticking  agents  because  of  their  versatility,  their  resistance  to 
solvents,  and  their  ability  to  develop  strong  and  reliable  joints. 

Phenolics.  Phenolics  are  characterized  by  their  low  cost  and  heat  resistance  of  up  to 
about  930°F  (500°C).  They  can  be  cured  by  a  hardener  or  by  heat  or  can  be  used  in 
solvents  that  evaporate  and  thus  allow  setting  to  occur.  Like  epoxies,  phenolics  are 
thermosetting  polymers  with  good  strength,  but  they  generally  suffer  from  brittleness. 

Polyamide.  The  polyamide  group  of  polymers  is  characterized  by  its  oil  and  water  re- 
sistance. Polyamides  are  usually  applied  in  the  form  of  hot  melts  but  can  also  be  used 
by  evaporation  of  solvents  in  which  they  have  been  dissolved.  Polyamides  are  nor- 
mally used  as  can-seam  sealants  and  the  like.  They  are  also  used  as  hot-melt  for  shoes. 

Silicones.  Silicones  can  perform  well  at  elevated  temperatures;  however,  cost  and 
strength  are  the  major  limitations.  Therefore,  silicones  are  usually  used  as  high- 
temperature  sealants. 

Joint  Preparation 

The  surfaces  to  be  bonded  must  be  clean  and  degreased  because  most  adhesives  do  not 
amalgamate  with  fats,  oils,  or  wax.  Joint  preparation  involves  grinding  with  sandpaper, 
grinding  and  filling,  sand  blasting,  and  pickling  and  degreasing  with  trichlorethylene. 
Oxide  films,  electroplating  coats,  and  varnish  films  need  not  be  removed  (as  long  as 
they  are  fixed  to  the  surface).  Roughening  of  the  surface  is  advantageous,  provided  that 
it  is  not  overdone. 

Joint  Design 

There  are  basically  three  types  of  adhesive-bonded  joints.  They  are  shown  in  Figure 
4.49  and  include  tension,  shear,  and  peel  joints.  Most  of  the  adhesives  are  weaker  in 
peel  and  tension  than  in  shear.  Therefore,  when  selecting  an  adhesive,  you  should  al- 
ways keep  in  mind  the  types  of  stresses  to  which  the  joint  is  going  to  be  subjected.  It 
is  also  recommended  that  you  avoid  using  tension  and  peel  joints  and  change  the  de- 
sign to  replace  these  by  shear  joints  whenever  possible. 


Tension 


Sheer  Peel 


130 


4      Joining  of  Metals 





Review  Questions 


» 


1.  What  does  the  riveting  process  involve? 

2.  What  are  rivets  usually  made  of? 

3.  List  some  applications  of  riveting. 

4.  Spot  welding  could  not  completely  replace  riv- 
eting. List  some  applications  of  riveting  that 
cannot  be  done  by  spot  welding. 

5.  How  would  you  define  welding? 

6.  List  five  types  of  welded  joint  designs  and  dis- 
cuss suitable  applications  for  each  type. 

7.  What  are  the  types  of  different  methods  for 
classifying  the  welding  processes? 

8.  How  would  you  break  all  the  manufacturing 
methods  into  groups  according  to  each  of 
these  classifying  methods? 

9.  Explain  briefly  the  mechanics  of  solid-state 
welding. 

10.  What  are  the  two  main  obstacles  that  must  be 
overcome  so  that  successful  pressure  welding 
can  be  achieved? 

11.  What  is  cold-pressure  welding?  Give  two  ex- 
amples, using  sketches. 

12.  Discuss  briefly  the  mechanics  of  explosive 
welding  and  draw  a  sketch  to  show  the  inter- 
face between  the  welded  parts. 

13.  List  some  industrial  applications  for  explosive 
welding. 

14.  Discuss  briefly  the  mechanics  of  ultrasonic 
welding. 

15.  What  are  the  typical  applications  of  ultrasonic 
welding? 

16.  What  are  the  different  types  of  ultrasonic 
welding  machines?  List  the  main  components 
common  to  all  these  machines. 

17.  What  is  the  basic  idea  on  which  friction  weld- 
ing is  based? 


18.  Explain  briefly  the  stages  involved  in  a  fric- 
tion welding  operation. 

19.  List  the  various  advantages  claimed  for  fric- 
tion welding. 

20.  List  the  limitations  of  friction  welding. 

21.  What  is  the  difference  between  friction  weld- 
ing and  inertia  welding? 

22.  What  advantages  does  inertia  welding  have 
over  friction  welding? 

23.  Give  examples  of  some  parts  that  are  fabri- 
cated by  inertia  welding. 

24.  Explain  briefly  the  mechanics  of  induction 
welding. 

25.  List  some  of  the  common  industrial  applica- 
tions of  the  induction  welding  process. 

26.  What  is  the  source  of  energy  in  thermit  weld- 
ing? Explain. 

27.  How  is  thermit  welding  performed? 

28.  What  are  the  common  applications  of  thermit 
welding? 

29.  How  does  bonding  take  place  in  diffusion 
bonding? 

30.  List  the  different  processes  that  belong  to  re- 
sistance welding. 

31.  Explain  briefly  the  stages  involved  in  a  resis- 
tance-butt welding  process. 

32.  Using  a  sketch,  explain  the  pressure-time  and 
current-time  relationships  in  resistance-butt 
welding. 

33.  List  some  of  the  applications  of  resistance-butt 
welding. 

34.  Clarify  the  difference  between  flash  welding 
and  butt  welding. 

35.  Draw  a  graph  illustrating  current  versus  time 
and  pressure  versus  time  in  flash  welding. 


Chapter  4  Review  Questions 


131 


36.  When  is  flash  welding  recommended  over  butt 
welding? 

37.  What  is  the  major  disadvantage  of  flash  weld- 
ing? 

38.  Explain  the  basic  idea  of  percussion  welding. 

39.  Explain  briefly  the  mechanics  of  spot  welding. 

40.  Draw  a  sketch  of  a  section  through  a  spot- 
welded  joint. 

41.  Draw  a  sketch  to  show  a  typical  cycle  for  a 
spot-welding  machine. 

42.  How  do  you  compare  seam  welding  with  spot 
welding? 

43.  List  some  of  the  advantages  of  seam  welding. 

44.  What  are  the  industrial  applications  of  seam 
welding? 

45.  What  is  the  basic  idea  of  projection  welding? 

46.  What  are  the  advantages  of  projection  weld- 
ing? 

47.  What  is  the  condition  for  two  metals  to  be 
joined  together  by  fusion  welding? 

48.  How  many  zones  can  be  identified  in  a  joint 
produced  by  a  conventional  fusion  welding 
process?  Discuss  briefly  the  microstructure  in 
each  of  these  zones. 

49.  For  what  do  the  letters  HAZ  stand? 

50.  Explain  briefly  the  phenomenon  of  the  electric 
arc  and  how  it  can  be  employed  in  welding. 

51.  What  are  the  advantages  of  alternating  current 
over  direct  current  in  arc  welding? 

52.  What  is  the  difference  between  DCSP  and 
DCRP?  When  would  you  recommend  using 
each  of  them? 

53.  What  is  meant  by  the  rated  duty  cycle? 

54.  What  shields  the  molten  metal  during 
shielded-metal  arc  welding? 

55.  What  is  the  main  shortcoming  of  shielded- 
metal  arc  welding? 

56.  List  some  of  the  functions  of  electrode  coat- 
ings. 


57.  Explain  briefly  the  Bernardos  welding  method. 

58.  What  is  the  main  feature  of  the  electrodes  in 
flux-cored  arc  welding? 

59.  What  provides  the  shielding  in  flux-cored  arc 
welding? 

60.  What  is  stud  arc  welding?  How  is  it  per- 
formed? List  the  main  applications  of  this 
process. 

61.  How  is  shielding  achieved  in  submerged  arc 
welding? 

62.  Why  must  the  plates  to  be  joined  by  sub- 
merged arc  welding  be  horizontal  only? 

63.  Why  does  submerged  arc  welding  always 
yield  very  high  quality  welds? 

64.  List  some  of  the  advantages  of  submerged  arc 
welding. 

65.  What  provides  shielding  in  MIG  welding? 

66.  Why  does  the  MIG  welding  process  render  it- 
self suitable  for  automation? 

67.  How  can  the  penetration  for  gas-metal  arc 
welding  be  controlled? 

68.  What  is  the  main  difference  between  the  MIG 
and  the  TIG  welding  processes? 

69.  List  some  of  the  applications  of  TIG  welding. 

70.  In  TIG  welding,  when  would  you  use  an  ac 
power  supply  and  when  would  you  use  DCSP 
and  DCRP? 

71.  Explain  briefly  the  mechanics  and  the  basic 
idea  of  plasma  arc  welding. 

72.  When  is  plasma  arc  welding  most  recom- 
mended? 

73.  Do  you  consider  electroslag  welding  to  be  a 
true  arc  welding  process? 

74.  How  does  welding  take  place  in  the  elec- 
troslag welding  process? 

75.  When  is  electroslag  welding  usually  recom- 
mended? 

76.  What  is  the  source  of  energy  in  oxyacetylene 
flame  welding? 


132 


4      Joining  of  Metals 


77.  How  is  acetylene  stored  for  use  in  welding 
operations? 

78.  What  does  the  equipment  required  in  gas 
welding  include?  Explain  the  function  of  each 
component. 

79.  What  are  the  types  of  flames  that  can  be  ob- 
tained in  gas  welding?  How  is  each  one  ob- 
tained? 

80.  What  are  the  zones  of  a  neutral  flame?  Discuss 
the  effect  of  the  oxygen-to-acetylene  ratio  on 
the  nature  of  the  flame  obtained. 

81.  Explain  briefly  the  operating  principles  of 
electron-beam  welding. 

82.  What  are  the  major  limitations  of  the  electron- 
beam  welding  process? 

83.  List  some  of  the  advantages  of  electron-beam 
welding. 

84.  What  are  the  major  applications  of  electron- 
beam  welding? 

85.  For  what  do  the  letters  in  the  word  laser 
stand? 

86.  Using  a  sketch,  explain  how  a  laser  beam  ca- 
pable of  carrying  out  welding  can  be  gener- 
ated. 

87.  Explain  briefly  the  mechanics  of  laser-beam 
welding. 

88.  What  are  the  main  advantages  of  laser-beam 
welding? 

89.  List  some  of  the  applications  of  laser-beam 
welding. 

90.  Using  sketches,  illustrate  the  commonly  expe- 
rienced welding  defects.  How  can  each  be 
avoided? 

91.  What  are  the  main  tests  for  the  inspection  of 
welds?  Discuss  each  briefly. 

92.  What  are  the  factors  affecting  the  selection  of 
the  joint  type? 

93.  On  what  basis  are  the  location  and  distribution 
of  welds  planned? 


94.  What  rules  would  you  consider  when  design- 
ing a  welded  joint? 

95.  What  is  meant  by  hard-facing  ? 

96.  What  are  the  main  applications  of  hard-fac- 
ing? 

97.  List  the  main  types  of  thermal  cutting 
processes.  Discuss  briefly  the  advantages  and 
limitations  of  each. 

98.  How  do  the  mechanics  of  brazing  differ  from 
those  of  welding? 

99.  What  is  the  main  difference  between  brazing 
and  soft  soldering? 

100.  List  some  of  the  alloys  used  as  brazing  fillers 
and  mention  the  base  metals  that  can  be 
brazed  with  each  one. 

101.  List  some  of  the  commonly  used  soft  solders. 

102.  What  is  the  main  function  of  brazing  fluxes? 

103.  List  some  of  the  fluxes  used  in  brazing. 

104.  List  some  of  the  fluxes  used  in  soft  soldering. 
Discuss  the  limitations  and  applications  of 
each. 

105.  In  soft  soldering,  how  should  the  solder  be 
fused? 

106.  List  the  different  brazing  techniques  used  in 
industry.  Discuss  the  advantages  and  limita- 
tions of  each. 

107.  As  a  product  designer,  what  factors  should 
you  take  into  consideration  when  designing  a 
brazed  joint? 

108.  In  what  case  can  sticking  of  metals  not  be  re- 
placed by  other  welding  and  brazing  tech- 
niques? 

109.  List  some  of  the  commonly  used  adhesives. 
Discuss  the  characteristics  and  common  appli- 
cations of  each. 

110.  What  are  the  types  of  adhesive-bonded  joints? 
Which  one  is  usually  the  strongest? 


Chapter  4  Design  Example 


133 


Problem 


3 


1.  Two  steel  slabs,  each  1/4  inch  (6.35  mm)  thick, 
are  to  be  joined  by  two  fillet  welds  (i.e.,  at  both 
edges).  If  the  width  of  each  slab  is  2.5  inches 
(62.5  mm)  and  the  joint  is  to  withstand  a  load  of 
35,000  pounds  (156,000  N),  determine  the  al- 
lowable tensile  strength  of  the  electrode  type  to 
be  used  in  welding. 

2.  Two  steel  plates,  each  1/4  inch  (6.35  mm)  thick, 
are  to  be  joined  by  two  fillet  welds.  If  the  joint  is 
to  withstand  a  load  of  50,000  pounds  (222,500  N) 
and  an  E7014  electrode  (allowable  tensile 
strength  =  21,000  lb/in.2  i.e.,  145,000  KN/m2)  is 
used,  determine  the  length  of  weld  at  each  edge. 
If  the  plate  width  is  10  inches  (254  mm),  how 
would  you  distribute  the  weld?  Draw  a  sketch. 

3.  Two  steel  plates,  each  5/16  inch  thick  (7.9  mm), 
are  to  be  fillet-welded  to  a  third  one  that  is  sand- 
wiched between  them.  The  width  of  each  of  the 
first  two  plates  is  4  inches  (100  mm),  whereas 
the  width  of  the  third  one  is  6  inches  (150  mm). 
The  two  plates  overlap  the  third  one  by  6  inches 
(150  mm),  and  an  E7014  electrode  (allowable 
tensile  strength  =  21,000  lb/in.2,  145,000 
KN/m2)  is  to  be  used.  If  the  joint  is  to  withstand 
a  load  of  190,000  pounds  (846  KN),  use  a  sketch 
to  illustrate  a  design  for  this  joint  and  provide  all 
calculations. 


An  equal-leg-angle  steel  section  3  by  3  by  1/4 
inch  (75  by  75  by  6  mm)  is  to  be  welded  to  a 
plate  using  an  E7014  electrode  (allowable  tensile 
strength  =  21,000  lb/in.2  i.e.,  145,000  KN/m2).  If 
the  joint  is  to  withstand  a  load  of  10,000  pounds 
(44.5  KN)  coinciding  with  the  axis  of  the  angle, 
design  the  joint  and  make  a  sketch  indicating  the 
distribution  of  the  weld  to  eliminate  any  ten- 
dency of  the  angle  to  rotate. 

Two  mild  steel  pipes,  each  having  a  3/4-inch 
(19  mm)  outer  diameter,  are  to  be  joined  to- 
gether by  brazing.  Assuming  that  the  joint  is  to 
withstand  an  axial  load  of  6  tons,  give  a  detailed 
design  of  this  joint.  (Take  allowable  shear  stress 
of  copper  to  be  6000  lb/in.2  i.e.,  41,430  KN/m2.) 

Two  mild  steel  sheets,  each  3/32  inch  (2.4  mm) 
thick,  are  to  be  brazed  together  using  copper  as  a 
filler  material.  Calculate  the  strength  of  the  joint 
when  it  is  manufactured  according  to  each  of  the 
designs  given  in  Figure  4.48.  Compare  the  re- 
sults and  recommend  the  design  that  gives  max- 
imum strength. 

A  power  supply  for  arc  welding  is  rated  at  a 
150-A  30-percent  duty  cycle.  What  will  be  the 
percentage  of  actual  time  utilized  in  welding  to 
the  total  time  the  power  supply  is  on  if  the  cur- 
rent employed  in  welding  is  only  125  A? 


Design 


r-  W&4TA 
Example 


PROBLEM 

You  are  required  to  design  a  flat-belt  pulley  so  that  it  can  be  fabricated  by  welding. 
The  pulley  is  to  be  mounted  on  a  shaft  that  is  1  lA  inches  (3 1  mm)  in  diameter,  and  the 
outside  diameter  of  the  rim  is  10  inches  (250  mm).  The  rim  of  the  pulley  is  to  provide 


134 


4      Joining  of  Metals 


a  surface  to  transmit  a  torque  of  600  lb  ft  (816  Nm)  from  a  2-inch- wide  (50-mm)  flat 
belt  to  the  shaft.  The  number  of  pulleys  required  is  only  5. 

Solution 

It  is  advisable  to  start  by  gathering  information  about  guidelines  for  the  construc- 
tional features  of  flat-belt  pulleys  (e.g.,  width  of  rim  for  a  certain  belt  width  and  thick- 
ness of  rim).  Information  about  the  safe  speeds  of  various  sizes  of  pulleys  should  also 
be  collected. 

Key.  The  best  strategy  is  to  design  the  key  so  that  it  will  be  the  weak  link  in  the 
pulley-key-shaft  assembly  because  it  is  easy  to  replace.  A  suitable  key  material  is 
AISI  1020  CD  steel,  which  is  commercially  available  as  a  key  stock  material.  It  has 
the  following  mechanical  properties: 

Ultimate  Tensile  Strength  (UTS)  =  78,000  lb/in.2 

yield  stress  =  66,000  lb/in." 

yield  stress  in  shear  =  38,000  lb/in.2 

Consider  Figure  4.50.  The  force  acting  on  the  key  is  given  by 

„      T      600  lb  ft  x  12  in/ft      ,,  c„n 

P  =  — =  1 1 ,520  pounds 

r  0.625  inch 

Take  the  key  cross  section  to  be  1/4  by  1/4  inch  (6  by  6  mm),  and  its  length  €  in  inches: 

.     .           11,520  pounds    . 
shear  stress  in  the  key  = <  xaii0wabie 

'/4X€ 

Take  a  safety  factor  of  2  for  the  key: 
38,000      , _  „„„  ,.  ..    2 

Xallowable  =  ~ ~ =    19,000  lb/in.~ 


FIGURE    4.50 

Forces  acting  on  the 
key 


0.  25  inch 


fl=  0.625  inch 


Chapter  4  Design  Example  135 


Therefore, 
11,520 


=  19.000 


!/4x€ 
and 

£  =  2.4  inches  (60  mm) 
But, 

11,520         ^     „  ui  ♦ 

bearing  stress  = <  allowable  compressive  stress 

!/4  xVixi 

<  66,000 

2 

=  33,000  lb/in.2  (safety  factor  of  2) 
Therefore, 

€  =  2.8  inches  (70  mm) 

We  take  this  value  to  ensure  safety  against  both  shearing  and  compressing  loads.  We 
should,  however,  round  it,  so  the  length  of  the  key  is  to  be  3  inches  (75  mm). 

Hub.  Use  a  round  seamless  tube  having  a  2.25-inch  outer  diameter  and  9/16-inch 
wall.  A  suitable  material  is  AISI  1020  CD  steel.  Again,  the  length  of  the  hub  must  not 
be  less  than  2.8  inches  to  keep  the  bearing  stress  below  the  allowable  value.  Take  it  as 
2.875  inches. 

Rim.  Use  a  round  seamless  tube  having  a  10-inch  outer  diameter  and  1/4-inch  wall. 
Again,  a  suitable  material  is  AISI  1020  CD  steel  because  of  its  availability  and  ability 
to  withstand  the  rubbing  effect  of  the  moving  belt. 

Spokes.  The  positioning  and  welding  of  four  or  five  spokes  would  create  a  serious 
problem  and  necessitates  the  use  of  a  complicated  welding  fixture.  Therefore,  the 
spokes  are  to  be  replaced  in  the  design  by  a  web.  Use  a  5/16-inch  flat  plate,  machined 
to  have  an  outer  diameter  of  9.43  inches  and  an  inner  diameter  2.31  inches.  An  appro- 
priate material  is  AISI  1020  HR  steel.  Because  weight  can  be  a  factor,  it  is  good  prac- 
tice to  provide  six  equally  spaced  holes  in  the  web  by  machining.  These  can  also  serve 
as  an  aid  in  the  handling  and  positioning  of  the  web  during  welding. 

Welding.  Use  conventional  arc  welding;  an  E7014  electrode  (allowable  tensile 
stress  =  21,000  lb/in.2)  can  be  used.  A  fillet  weld  with  a  leg  of  1/4  inch  is  adopted.  The 
force  is  given  by 

p  =  torque  =         7200         =  ^  pounds 
radius       1.125  inches 


c 

H  £ 

"'  o 

■  .c 

t  « 

UJ  | 

O  to 

—  ■£ 

LL  Q 


136 


Chapter  4  Design  Projects  137 


The  required  length  of  the  weld  is 
6400 


lAx  0.707x0.57x21,000 


=  3.03  inches  (75  mm) 


Use  2.00  inches  (50  mm)  of  weld  on  each  side  of  the  hub. 

The  circumference  of  the  hub  equals  n  times  2.25,  or  7.06  inches.  Space  four 
welds,  each  0.5  inch  (12.5  mm)  in  length,  equally,  90°  apart  around  the  circumference 
of  the  hub.  Welds  on  both  sides  of  the  web  should  be  staggered.  Adopt  the  same  welds 
at  the  rim.  They  should  be  safe  because  the  shearing  force  is  much  lower  (the  radius  is 
larger  than  that  of  the  hub). 

Once  all  the  dimensions  and  details  are  known,  we  are  in  a  position  to  construct 
the  pulley  as  shown  in  the  workshop  drawing  in  Figure  4.51. 


jgn  Projects 


1.  Design  a  table  for  the  machine  shop.  The  table  should  be  4  feet  (1200  mm)  in 
height,  with  a  surface  area  of  3  by  3  feet  (900  by  900  mm),  and  should  be  able  to 
carry  a  load  of  half  a  ton.  Because  only  two  tables  are  required,  the  design  should 
involve  the  use  of  steel  angles  and  a  plate  that  are  to  be  joined  together  by  welding. 

2.  Design  a  tank  for  compressed  air.  It  has  a  capacity  of  100  cubic  feet  (2.837  m3)  and 
can  withstand  an  internal  pressure  of  40  atmospheres  (ata).  The  number  of  tanks  re- 
quired is  50,  and  the  tanks  are  going  to  be  placed  in  a  humid  environment. 

3.  Design  a  compressed-air  reservoir  (tank)  that  is  to  be  subjected  to  an  extremely  cor- 
rosive environment.  The  capacity  of  the  tank  is  30  cubic  feet  (0.85  m3),  and  the 
maximum  gage  pressure  is  70  ata,  but  the  pressure  is  pulsating  from  zero  to  the 
maximum  value  about  once  every  5  minutes.  The  number  of  tanks  required  is  100. 

4.  A  straight-toothed  spur-gear  wheel  transmits  a  torque  of  1200  lb  ft  (1632  Nm)  to  a 
2-inch-diameter  (50  mm)  steel  shaft  (AISI  1045  CD  steel).  The  pitch  diameter  of 
the  gear  is  8  inches  (200  mm),  its  width  is  3  inches  (75  mm),  and  the  base  diame- 
ter is  7.5  inches  (187.5  mm).  Make  a  detailed  design  for  the  gear's  blank  (i.e.,  be- 
fore the  teeth  are  cut). 

5.  A  mobile  winch  (little  crane)  can  be  moved  on  casters.  It  has  a  capacity  of  lifting 
1  ton  for  3  feet  (0.9  m)  about  ground.  The  lifting  arm  can  be  extended,  and  the 
winch  can  then  lift  1/2  ton  for  up  to  6  feet  (1.8  m).  Knowing  that  the  production 
volume  is  4000  units  and  that  casters  and  hydraulic  pressure  cylinders  are  to  be  pur- 
chased from  vendors,  provide  a  detailed  design  and  include  full  specifications  of 
the  parts  to  be  purchased. 


138  4      Joining  of  Metals 


6.  The  lifting  arm  for  a  crane  is  60  feet  (about  20  m),  and  its  lifting  capacity  is  1  ton. 
It  is  to  be  used  in  construction  work  and  to  be  subjected  to  humidity,  dirt,  and  so 
on.  Provide  a  detailed  design  for  this  arm  using  steel  angles  that  are  to  be  welded 
together. 

7.  Design  a  frame  for  a  hydraulic  press  for  fabrication  by  welding.  The  height  of  the 
cross  arm  is  12  feet  (about  4  m).  The  cross  arm  is  mounted  (by  welding)  on  two 
vertical  columns  that  are,  in  turn,  welded  to  the  base.  The  press  can  produce  a  max- 
imum load  of  200  tons  by  means  of  a  hydraulic  cylinder  attached  to  the  cross  arm 
(below  it),  and  the  stroke  is  12  inches  (300  mm). 

TIP:  The  energy  absorbed  when  the  frame  deforms  should  not  exceed  2  per- 
cent of  the  total  energy  output  of  the  press. 


Chapter  5 


Metal  Forming 


T 


-+■ 


INTRODUCTION 

Metal  forming  processes  have  gained  significant  attention  since  World  War  II  as 
a  result  of  the  rapid  increase  in  the  cost  of  raw  materials.  Whereas  machining 
processes  involve  the  removal  of  portions  of  the  stock  material  (in  the  form  of 
chips)  in  order  to  achieve  the  required  final  shape,  metal  forming  processes 
are  based  upon  the  plastic  deformation  and  flow  of  the  billet  material  in  its 
solid  state  so  as  to  take  the  desired  shape.  Consequently,  metal  forming 
processes  render  themselves  more  efficient  with  respect  to  raw  material  uti- 
lization than  machining  processes,  which  always  result  in  an  appreciable  ma- 
terial waste. 

In  fact,  although  metal  forming  techniques  were  employed  in  manufacturing 
only  semifinished  products  (like  sheets,  slabs,  and  rods)  in  the  past,  finished 
products  that  require  no  further  machining  can  be  produced  today  by  these 
techniques.  This  was  brought  about  by  the  recent  developments  in  working 
methods,  as  well  as  by  the  construction  features  of  the  forming  machines  em- 
ployed. Among  the  advantages  of  these  up-to-date  forming  techniques  are  high 
productivity  and  very  low  material  waste.  Therefore,  more  designers  tend  to 
modify  the  construction  of  the  products  manufactured  by  other  processes  to 
use  forming.  Also,  bearing  in  mind  that  metal  forming  methods  are  still  being 
used  for  producing  semifinished  products,  it  is  evident  that  the  vast  majority  of 
all  metal  products  are  subjected  to  forming,  at  least  at  one  stage  during  their 
production.  This  latter  fact  clearly  manifests  the  importance  of  the  metal  form- 
ing methods. 


139 


140  5      Metal  Forming 


Generally,  metal  forming  involves  both  billet  and  sheet  metal  forming.  How- 
ever, it  has  been  a  well-accepted  convention  to  divide  those  processes  into  two 
main  groups:  bulk  (or  massive)  forming  and  sheet  metal  working.  In  this  chap- 
ter, only  bulk  forming  processes  (e.g.,  forging,  cold  forming,  and  rolling)  are 
covered;  Chapter  6  deals  with  the  working  of  sheet  metal. 


i6> 


LASTIC  DEFORMATION 


Factors  Affecting  Plastic  Deformation 

During  any  forming  process,  the  material  plastically  flows  while  the  total  volume  of 
the  workpiece  remains  substantially  constant.  However,  there  are  some  marked 
changes  that  take  place  on  a  microscopic  scale  within  the  grains  and  the  crystal  lattice 
of  the  metal,  resulting  in  a  corresponding  change  in  the  properties  of  the  material.  This 
latter  change  can  be  explained  in  view  of  the  dislocation  theory,  which  states  that  the 
plastic  deformation  and  flow  of  metal  are  caused  by  movement  and  transfer  of  dislo- 
cations (defects  in  the  crystal  lattice)  through  the  material  with  the  final  outcome  of  ei- 
ther piling  up  or  annihilating  them.  Following  are  some  factors  that  affect  plastic 
deformation  by  influencing  the  course  of  dislocations. 

Impurities  and  alloying  additives.  It  is  well  known  that  pure  metals  possess  higher 
plasticity  than  their  alloys.  The  reason  is  that  the  presence  of  structural  components 
and  chemical  compounds  impedes  the  transfer  and  migration  of  dislocations,  resulting 
in  lower  plasticity. 

Temperature  at  which  deformation  takes  place.  As  a  rule,  the  plasticity  of  a  metal 
increases  with  temperature,  whereas  its  resistance  to  deformation  decreases.  The 
higher  the  temperature,  the  higher  the  plasticity  and  the  lower  the  yield  point.  More- 
over, no  work-hardening  occurs  at  temperatures  above  the  recrystallization  tempera- 
ture. This  should  be  expected  because  recrystallization  denotes  the  formation  and 
growth  of  new  grains  of  metal  from  the  fragments  of  the  deformed  grains,  together 
with  restoring  any  distortion  in  the  crystal  lattice.  Consequently,  strength  values  drop 
to  the  level  of  a  nonwork-hardened  state,  whereas  plasticity  approaches  that  of  the 
metal  before  deformation.  In  fact,  a  forming  process  is  termed  hot  if  the  tempera- 
ture at  which  deformation  takes  place  is  higher  than  the  recrystallization  temperature. 
Lead  that  is  formed  at  room  temperature  in  summer  actually  undergoes  hot  forming 
because  the  recrystallization  temperature  for  lead  is  39.2°F  (4°C).  When  deforma- 
tion occurs  at  a  temperature  below  the  recrystallization  temperature  of  the  metal,  the 
process  is  termed  cold  forming.  Cold  forming  processes  are  always  accompanied  by 
work-hardening  due  to  the  piling  up  of  dislocations.  As  a  result,  strength  and  hardness 
increase  while  both  ductility  and  notch  toughness  decrease.  These  changes  can  be 
removed  by  heat  treatment  (annealing).  On  the  other  hand,  when  hot  forming  a 
metal,  the  initial  dendritic  structure  (the  primary  structure  after  casting)  disintegrates 
and  deforms,  and  its  crystals  elongate  in  the  direction  of  the  metal  flow.  The  insoluble 


5.1  Plastic  Deformation 


141 


impurities  like  nonmetallic  inclusions  (around  the  original  grain  boundaries)  are  drawn 
and  squeezed  between  the  elongated  grains.  This  texture  of  flow  lines  is  usually  re- 
ferred to  as  the  fibrous  macrostructure.  This  fibrous  macrostructure  is  permanent  and 
cannot  be  removed  by  heat  treatment  or  further  working.  As  a  result,  there  is  always 
anisotropy  of  mechanical  properties;  strength  and  toughness  are  better  in  the  longitu- 
dinal direction  of  fibers.  Also,  during  hot  forming,  any  voids  or  cracks  around  grain 
boundaries  are  closed,  and  the  metal  welds  together,  which,  in  turn,  results  in  im- 
provements in  the  mechanical  properties  of  the  metal. 

Rate  of  deformation.  It  can  generally  be  stated  that  the  rate  of  deformation  (strain 
rate)  in  metal  working  adversely  affects  the  plasticity  of  the  metal  (i.e.,  an  increase  in 
the  deformation  rate  is  accompanied  by  a  decrease  in  plasticity).  Because  it  takes  the 
process  of  recrystallization  some  time  to  be  completed,  that  process  will  not  have 
enough  time  for  completion  when  deformation  occurs  at  high  strain  rates.  Therefore, 
greater  resistance  of  the  metal  to  deformation  should  be  expected.  This  does  not  mean 
that  the  metal  becomes  brittle. 

State  of  stress.  A  state  of  stress  at  a  point  can  be  simply  described  by  the  magni- 
tudes and  directions  of  the  principal  stresses  (a  stress  is  a  force  per  unit  area)  acting  on 
planes  that  include  the  point  in  question.  The  state  of  stress  is,  in  fact,  a  precise  and 
scientific  expression  for  the  magnitudes  and  the  directions  of  the  external  forces  acting 
on  the  metal.  All  possible  states  of  stress  can  be  reduced  to  only  nine  main  systems,  as 
shown  in  Figure  5.1.  These  nine  cases  can,  in  turn,  be  divided  into  three  groups.  The 


FIGURE    5.1 

The  nine  main  systems 
of  the  state  of  stress 


1 

,\ 

a) 

7 

/!' 

/ 

/"l 

7 

i 
? — 

7 

Uniaxial 


J— 

'     i 


<A 


.o2     Plane 


// 


s 


"a 


»_    II.,  On -*- 


a7      o2- 


Triaxial 
(solid) 


142 


Metal  Forming 


first  group  includes  two  systems  that  are  characterized  by  the  absence  of  stress  (forces) 
along  two  directions,  and  the  stress  system  is  therefore  called  uniaxial.  This  is  the  case 
when  stretching  sheet  metal  having  a  length  that  considerably  exceeds  its  width.  In 
each  of  the  three  systems  included  in  the  second  group,  it  is  clear  that  a  stress  along 
only  one  of  the  directions  is  absent.  Because  the  other  two  directions  (stresses)  form  a 
plane,  each  of  these  systems  is  referred  to  as  a  plane-stress  state.  It  may  approximately 
be  represented  by  stretching  of  a  thin  sheet  in  two  or  more  directions.  The  remaining 
group  indicates  the  state  of  stress  of  a  body,  where  there  are  stresses  acting  along  all 
three  directions  in  space,  yielding  the  term  triaxial.  In  fact,  most  of  the  bulk  forming 
operations  (forging,  rolling,  and  wire  drawing)  cause  states  of  stress  that  belong  to  this 
latter  group. 

Load  and  Energy  Requirement 

The  force  required  for  deforming  a  given  metal  (at  any  unchanged  desired  temperature 
and  at  usual  strain-rate  levels)  is  dependent  upon  the  degree  of  deformation,  which  is 
the  absolute  value  of  the  natural  logarithm  of  the  ratio  of  the  final  length  of  the  billet 
to  its  original  length.  On  the  other  hand,  the  energy  consumed  throughout  the  forming 
process  is  equivalent  to  the  area  under  the  load-deformation  curve  for  that  forming 
process.  Therefore,  that  energy  can  be  calculated  if  the  relationship  between  the  load 
and  the  deformation  is  known.  Figure  5.2  shows  the  degree  of  deformation  and  the  en- 
ergy consumed  in  an  upsetting  operation.  It  must  be  noted  that  in  both  hot  and  cold 
working,  there  is  an  upper  limit  for  the  degree  of  deformation  (especially  in  cold  work- 
ing) above  which  cracks  and  discontinuities  in  the  workpiece  initiate. 

Preheating  the  Metal  for  Hot  Forming 

Before  being  subjected  to  hot  forming  processes,  ingots  (or  billets)  should  be  uni- 
formly heated  throughout  their  cross  sections,  without  overheating  or  burning  the 
metal  at  the  surface.  This  is  particularly  important  when  forming  steels.  Attention  must 
also  be  given  to  the  problems  of  decarburization  and  formation  of  scale  in  order  to 
bring  them  to  a  minimum.  The  thermal  gradient  is  another  important  factor  that  affects 


FIGURE    5.2 

The  degree  of 
deformation  and  the 
energy  consumed  in 
upsetting 


l«4 


. I    — . — 

I       Original 
length 


Final 
length 


Deformation 


5.1  Plastic  Deformation  143 

the  soundness  of  the  deformed  part.  If  the  temperature  gradient  is  high,  thermal 
stresses  may  initiate  and  can  cause  internal  cracks.  This  usually  happens  when  a  por- 
tion of  the  metal  is  above  the  critical  temperature  of  metal  (AC]  or  AC3)  while  the  rest 
of  the  billet  is  not.  The  larger  the  cross  section  of  the  billet  and  the  lower  its  coefficient 
of  thermal  conductivity,  the  steeper  the  temperature  gradient  will  be,  and  the  more  li- 
able to  internal  cracking  during  heating  the  billet  becomes.  In  the  latter  case,  the  rate 
of  heating  should  be  kept  fairly  low  (about  2  hours  per  inch  of  section  of  the  billet)  in 
order  not  to  allow  a  great  difference  to  occur  between  the  temperatures  at  the  surface 
and  the  core  of  the  billet.  The  metal  must  then  be  "soaked"  at  the  maximum  tempera- 
ture for  a  period  of  time  long  enough  to  ensure  uniformity  of  temperature. 

The  maximum  temperature  to  which  the  billet  is  heated  before  forming  differs 
for  different  metals.  There  is  usually  an  optimum  range  of  temperatures  within  which 
satisfactory  forming  is  obtained  because  of  increased  plasticity  and  decreased  resis- 
tance to  deformation.  Nevertheless,  any  further  increase  in  temperature  above  that 
range  may,  on  the  contrary,  result  in  a  defective  product.  Burned  metal  and  coarse 
grain  structure  are  some  of  the  defects  encountered  when  a  metal  is  excessively 
heated. 

The  ingots  may  be  heated  in  soaking  pits,  forge  hearths,  chamber  furnaces,  or 
car-bottom  furnaces,  which  are  all  heated  with  gas.  Rotary  hearth  furnaces  represent 
another  type  of  heating  furnace.  In  mass  production  or  automated  lines,  small  ob- 
jects (billets)  are  heated  using  electric  current  and  the  phenomenon  of  induction. 
This  induction-heating  method  is  quick  and  keeps  the  surfaces  of  the  billets  clean, 
and  temperatures  can  be  accurately  controlled.  Moreover,  physical  equipment  re- 
quires limited  floor  space  and  can  be  fully  automated. 

Friction  and  Lubrication 
in  Working  of  Metals 

Friction  plays  an  important  role  in  all  metal  forming  processes  and  is  generally  con- 
sidered to  be  undesirable  because  it  has  various  harmful  effects  on  the  forming 
processes,  on  the  properties  of  products,  and  on  the  tool  life.  During  the  deformation 
of  a  metal,  friction  occurs  at  the  contact  surface  between  the  flowing  metal  and  the  tool 
profile.  Consequently,  the  flow  of  the  metal  is  not  homogeneous,  which  leads  to  the 
initiation  of  residual  stresses,  with  the  final  outcome  being  an  unsound  product  with  in- 
ferior surface  quality.  Also,  friction  increases  the  pressure  acting  on  the  forming  tool 
(as  well  as  the  power  and  energy  consumed)  and  thus  results  in  greater  wear  of  the 
tools. 

Friction  in  metal  forming  is  drastically  different  from  the  conventional  Columb's 
friction  because  extremely  high  pressure  between  the  mating  bodies  (tool  and  work- 
piece)  is  involved.  Recent  theories  on  friction  in  metal  forming  indicate  that  it  is  actu- 
ally the  resistance  to  shear  of  a  layer,  where  intensive  shear  stress  is  generated  as  a 
result  of  relative  displacement  between  two  bodies.  When  these  bodies  have  direct 
metal-to-metal  contact,  slipping  and  shear  flow  occur  in  a  layer  adjacent  to  the  contact 
interface.  But,  if  a  surface  of  contact  is  coated  with  a  material  having  low  shear  resis- 
tance (a  lubricant  that  can  be  solid  or  liquid),  slipping  takes  place  through  that  layer  of 


144 


5      Metal  Forming 


lubricant  and,  therefore,  has  low  resistance.  This  discussion  indicates  clearly  that  the 
magnitude  of  the  friction  force  is  determined  by  the  mechanical  properties  (yield  point 
in  shear)  of  the  layer  where  actual  slipping  occurs.  Hence,  it  is  evident  that  a  metal 
having  a  low  yield  point  in  shear,  such  as  lead,  can  be  used  as  a  lubricant  when  form- 
ing metals  having  relatively  high  yield  strength  in  compression.  Figure  5.3  shows  the 
shear  layer  in  three  different  cases:  solid  lubrication,  dry  sticking  friction,  and  hydro- 
dynamic  (liquid)  lubrication. 

In  order  to  reduce  friction  and  thus  eliminate  its  harmful  effects,  lubricants  are  ap- 
plied to  the  tool-workpiece  interface  in  metal  forming  processes.  The  gains  include 
lower  load  and  energy  requirement,  prevention  of  galling  or  sticking  of  the  workpiece 
metal  onto  the  tool,  better  surface  finish  of  products,  and  longer  tool  life.  An  important 
consideration  when  selecting  a  lubricant  is  its  activity  (i.e.,  its  ability  to  adhere 
strongly  to  the  surface  of  the  metal).  The  activity  of  a  lubricant  can,  however,  be  en- 
hanced by  adding  material  with  high  capability  of  adsorption,  such  as  fat  acids.  Among 
other  factors  to  be  considered  are  thermal  stability,  absence  of  poisonous  fumes,  and 
complete  burning  during  heat  treatment  of  the  products. 

In  cold  forming  processes,  vegetable  and  mineral  oils  as  well  as  aqueous  emul- 
sions are  employed  as  lubricants.  These  have  the  advantage  of  acting  as  coolants,  elim- 
inating excessive  heat  and  thus  reducing  the  temperature  of  the  tool.  Solid  polymers, 
waxes,  and  solid  soaps  (sodium  or  calcium  stearates)  are  also  widely  used  in  cold- 
metal  working. 

For  relatively  high  temperature  applications,  chlorinated  organic  compounds  and 
sulfur  compounds  are  used.  Solid  lubricants  like  molybdenum  disulfide  and  graphite 
possess  low-friction  properties  up  to  elevated  temperatures  and  are,  therefore,  used  as 
solid  lubricants  in  hot  forming.  Graphite  is  sometimes  dispersed  in  grease,  especially 
in  hot  forging  ferrous  materials.  Lately,  use  has  been  made  of  molten  glass  as  a  lubri- 
cant when  alloy  steels  and  special  alloys  are  hot  formed.  The  glass  is  added  in  the  form 
of  powder  between  the  die  and  a  hot  billet.  The  advantages  of  molten  glass  include  low 
friction,  excellent  surface  finish,  and  improved  tool  life. 


FIGURE    5.3 

The  shear  layer  in  three 

different  cases  Soft 

metal 


Pressure 


Dry  sticking 
friction 


Liquid 
lubricant 


5.2  ROLLING 


5.2  Rolling  145 

Cold  Forming  Versus  Hot  Forming 

Cold  forming  has  its  own  set  of  advantages  and  disadvantages,  as  does  hot  form- 
ing, and,  therefore,  each  renders  itself  appropriate  for  a  certain  field  of  applications. 
For  instance,  cold  forming  will  enhance  the  strength  of  the  workpiece  metal,  im- 
prove the  quality  of  the  surface,  and  provide  good  dimensional  accuracy,  but  the 
plastic  properties  of  the  metal  (elongation  percentage  and  reduction-in-area  percent- 
age) and  the  impact  strength  drop.  Therefore,  the  final  properties  of  cold-formed 
products  are  obtained  as  required  by  adjusting  the  degree  of  deformation  and  the 
parameters  of  the  postheating  treatment  process.  Because  the  loads  involved  in  cold 
forming  are  high,  this  technique  is  generally  employed  in  the  manufacture  of  small 
parts  of  soft,  ductile  metals,  such  as  low-carbon  steel.  Also,  large  quantities  must  be 
produced  to  justify  the  high  cost  of  tooling  involved.  Nevertheless,  if  the  products 
are  to  be  further  processed  by  machining,  the  increased  hardness  caused  by  cold 
working  is  a  real  advantage  because  it  results  in  better  machinability.  Therefore, 
cold-rolled  plates  and  cold-drawn  bars  are  more  suitable  for  machining  purposes 
than  hot-formed  ones. 

On  the  other  hand,  the  yield  strength  of  a  metal  drops  significantly  at  elevated 
temperatures,  and  no  work-hardening  occurs.  Consequently,  hot  forming  processes 
are  used  when  high  degrees  of  deformation  are  required  and/or  when  forming  large 
ingots  or  billets  because  the  loads  and  energies  needed  are  far  lower  than  those  re- 
quired in  cold  forming.  Moreover,  hot  forming  refines  the  grain  structure,  thus  pro- 
ducing softer  and  more  ductile  parts  suitable  for  further  processing  by  cold  forming 
processes.  However,  high  temperatures  affect  the  surface  quality  of  products,  giving 
oxidation  and  scales. 

Decarburization  may  also  occur  in  steels,  especially  when  hot  forming 
high-carbon  steel.  The  scales,  oxides,  and  decarburized  layers  must  be  removed  by 
one  or  more  machining  processes.  This  slows  down  the  production,  adds  machin- 
ing costs,  and  yields  waste  material,  resulting  in  lower  efficiency  of  material  uti- 
lization. A  further  limitation  of  hot  forming  is  reduced  tool  life  due  to  the 
softening  of  tool  surfaces  at  elevated  temperatures  and  the  rubbing  action  of  the 
hot  metal  while  flowing.  This  actually  subjects  the  tools  to  thermal  fatigue,  which 
shortens  their  life. 


Hot  rolling  is  the  most  widely  used  metal  forming  process  because  it  is  employed  to 
convert  metal  ingots  to  simple  stock  members  called  blooms  and  slabs.  This  process 
refines  the  structure  of  the  cast  ingot,  improves  its  mechanical  properties,  and  elimi- 
nates the  hidden  internal  defects.  The  process  is  termed  primary  rolling  and  is  fol- 
lowed by  further  hot  rolling  into  plates,  sheets,  rods,  and  structural  shapes.  Some  of 
these  may  be  subjected  to  cold  rolling  to  enhance  their  strength,  obtain  good  surface 
finish,  and  ensure  closer  dimensional  tolerances.  Figure  5.4  illustrates  the  sequence  of 
operations  involved  in  manufacturing  rolled  products. 


FIGURE    5.4 

Sequence  of  operations  involved  in  manufacturing  rolled  products 


Finished  products 


146 


5.2  Rolling 


147 


Principles  of  Rolling 

The  process  of  rolling  consists  of  passing  the  metal  through  a  gap  between  rolls  rotat- 
ing in  opposite  directions.  That  gap  is  smaller  than  the  thickness  of  the  part  being 
worked.  Therefore,  the  rolls  compress  the  metal  while  simultaneously  shifting  it  for- 
ward because  of  the  friction  at  the  roll-metal  interfaces.  When  the  workpiece  com- 
pletely passes  through  the  gap  between  the  rolls,  it  is  considered  fully  worked.  As  a 
result,  the  thickness  of  the  work  decreases  while  its  length  and  width  increase.  How- 
ever, the  increase  in  width  is  insignificant  and  is  usually  neglected.  As  can  be  seen  in 
Figure  5.5,  which  shows  the  rolling  of  a  plate,  the  decrease  in  thickness  is  called  draft, 
whereas  the  increase  in  length  and  the  increase  in  width  are  termed  absolute  elonga- 
tion and  absolute  spread,  respectively.  Two  other  terms  are  the  relative  draft  and  the 
coefficient  of  elongation,  which  can  be  given  as  follows: 


relative  draft  e  = 


Ah  x  100      K-  h{ 


x  100 


(5.1) 


coefficient  of  elongation  r\  =  — 
But  because  the  volume  of  the  work  is  constant,  it  follows  that 


n 


hn  x  bc 


At 


(5.2) 


(5.3) 


hf  x  bt 

Equation  5.3  indicates  that  the  coefficient  of  elongation  is  adversely  proportional  to  the 
ratio  of  the  final  to  the  original  cross-sectional  areas  of  the  work. 

As  can  be  seen  in  Figure  5.6,  the  metal  is  deformed  in  the  shaded  area,  or  defor- 
mation zone.  The  metal  remains  unstrained  before  this  area  and  does  not  undergo  any 
further  deformation  after  it.  It  can  also  be  seen  that  the  metal  undergoing  deformation 
is  in  contact  with  each  of  the  rolls  along  the  arc  AB,  which  is  called  the  arc  of  contact. 
It  corresponds  to  a  central  angle,  a,  that  is,  in  turn,  called  the  angle  of  contact,  or  angle 
of  bite.  From  the  geometry  of  the  drawing  and  by  employing  simple  trigonometry,  it 
can  be  shown  that 


cos  a  =  1 


K  -  hf  __  x  _  Ah 
2R  2R 


(5.4) 


FIGURE    5.5 

Simple  rolling  of  a  plate 


Neck 


Body 


Wobbler 


148 


5      Metal  Forming 


FIGURE    5.6 

The  deformation  zone, 
state  of  stress,  and 
angle  of  contact  in 
rolling 


Equation  5.4  gives  the  relationship  between  the  geometrical  parameters  of  the  rolling 
process,  the  angle  of  contact,  the  draft,  and  the  radius  of  the  rolls.  Note  that  in  order  to 
ensure  that  the  metal  will  be  shifted  by  friction,  the  angle  of  contact  must  be  less  than 
fi,  the  angle  of  friction,  where  tan  P  =  u,  (the  coefficient  of  friction  between  roll  surface 
and  metal).  In  fact,  the  maximum  permissible  value  for  the  angle  of  contact  depends 
upon  other  factors,  such  as  the  material  of  the  rolls,  the  work  being  rolled,  and  the 
rolling  temperature  and  speed.  Table  5.1  indicates  the  recommended  maximum  angle 
of  contact  for  different  rolling  processes. 

Load  and  Power  Requirement 

As  can  also  be  seen  in  Figure  5.6,  the  main  stress  system  in  the  deformation  zone  in  a 
rolling  process  is  triaxial  compression,  with  the  maximum  (principal)  stress  acting  nor- 
mal to  the  direction  of  rolling.  The  deformed  metal  is  exerting  an  equal  counterforce 
on  each  of  the  rolls  to  satisfy  the  equilibrium  conditions.  Therefore,  this  force  normal 
to  the  direction  of  rolling  is  important  when  doing  the  design  calculations  for  the  rolls 
as  well  as  the  mill  body.  It  is  also  important  in  determining  the  power  consumption  in 


TABLE    5.1 

Maximum  allowable 
angle  of  contact  for 
rolling 


Rolling  Process 


Maximum  Allowable 
Angle  of  Contact 


Rolling  of  blooms  and  heavy  sections 
Hot  rolling  of  sheets  and  strips 
Cold  rolling  of  lubricated  sheets 


24°-30c 

15°-20e 

2°-10c 


5.2  Rolling  149 

a  rolling  process.  Unfortunately,  the  exact  determination  of  that  rolling  load  and  power 
consumption  is  complicated  and  requires  knowledge  of  theory  of  plasticity  as  well  as 
calculus.  Nevertheless,  a  first  approximation  of  the  roll  load  can  be  given  by  the  fol- 
lowing simple  equation: 

F=Y  xbx  V«xM  <5-5) 

where  Y  is  the  average  (plane-strain)  yield  stress  assuming  no  spread  and  is  equal  to 
\.\5Y,  where  Y  is  the  mean  yield  stress  of  the  metal.  Therefore,  Equation  5.5  should 
take  the  form 

F=  l.\5YxbxVRxAh  (5-6) 

Equation  5.6  neglects  the  effect  of  friction  at  the  roll-work  interface  and,  therefore, 
gives  lower  estimates  of  the  load.  Based  on  experiments  carried  out  on  a  wide  range 
of  rolling  mills,  this  equation  can  be  modified  to  account  for  friction  by  multiplying  by 
a  factor  of  1 .2.  The  modified  equation  is 

F=  1.2  x  1.157x6  xV/TxA/i  (5.7) 

The  power  consumed  in  the  process  cannot  be  obtained  easily;  however,  a  rough  esti- 
mate in  low-friction  conditions  is  given  by 

Y  xbxRx  Ahxoi  ,ce, 

hp  = _ (5.8, 

where  co  is  the  angular  velocity  of  rolls  in  radians  per  second,  and  Y,  b,  R  and  Ah  are 
all  in  English  units. 

Rolling  Mills 

A  rolling  mill  includes  one  or  more  roll  stands  and  a  main  drive  motor,  reducing  gear, 
stand  pinion,  flywheel,  and  coupling  gear  between  the  units.  The  roll  stand  is  the  main 
part  of  the  mill,  where  the  rolling  process  is  actually  performed.  It  basically  consists  of 
housings  in  which  antifriction  bearings  that  are  used  for  carrying  (mounting)  the  rolls 
are  fitted.  Moreover,  there  is  a  screw-down  mechanism  to  control  the  gap  between  the 
rolls  and  thus  the  required  thickness  of  the  product. 

Depending  upon  the  profile  of  the  rolled  product,  the  body  of  the  roll  may  be  ei- 
ther smooth  for  rolling  sheets  (plates  or  strips)  or  grooved  for  manufacturing  shapes 
such  as  structural  members.  A  roll  consists  of  a  body,  two  necks  (one  on  each  side), 
and  two  wobblers  (see  Figure  5.5).  The  body  is  the  part  that  contacts  and  deforms  the 
metal  of  the  workpiece.  The  necks  rotate  in  bearings  that  act  as  supports,  while  the 
wobblers  serve  to  couple  the  roll  to  the  drive.  Rolls  are  usually  made  from  high- 
quality  steel  and  sometimes  from  high-grade  cast  iron  to  withstand  the  very  severe  ser- 
vice conditions  to  which  the  rolls  are  subjected  during  the  rolling  process,  such  as 
combined  bending  and  torque,  friction  and  wear,  and  thermal  effects.  Gray  cast-iron 
rolls  are  employed  in  roughing  passes  when  hot  rolling  steel.  Cast-  or  forged-steel  rolls 
are  used  in  blooming,  slabbing,  and  section  mills  as  well  as  in  cold-rolling  mills. 
Forged  rolls  are  stronger  and  tougher  than  cast  rolls.  Alloy-steel  rolls  made  of  chrome- 
nickel  or  chrome-molybdenum  steels  are  used  in  sheet  mills. 


150 


5      Metal  Forming 


Classification  of  Rolling  Mills 

Rolling  mills  are  classified  according  to  the  number  and  arrangement  of  the  rolls  in  a 
stand.  Following  are  the  five  main  types  of  rolling  mills,  as  shown  in  Figure  5.7a 
through  e. 

Two-high  rolling  mills.  Two-high  rolling  mills,  the  simplest  design,  have  a  two-high 
stand  with  two  horizontal  rolls.  This  type  of  mill  can  be  nonreversing  (unidirectional), 
where  the  rolls  have  a  constant  direction  of  rotation,  or  reversing,  where  the  rotation 
and  direction  of  metal  passage  can  be  reversed. 

Three-high  rolling  mills.  Three-high  rolling  mills  have  a  three-high  stand  with  three 
rolls  arranged  in  a  single  vertical  plane.  This  type  of  mill  has  a  constant  direction  of 
rotation,  and  it  is  not  required  to  reverse  that  direction. 

Four-high  rolling  mills.  In  sheet  rolling,  the  rolls  should  be  designed  as  small  as  pos- 
sible in  order  to  reduce  the  rolling  force  F  of  the  metal  on  the  rolls  and  the  power  re- 
quirement. If  such  small-diameter  rolls  are  used  alone,  they  will  bend  and  result  in 
nonuniform  thickness  distribution  along  the  width  of  the  sheet,  as  shown  in  Figure  5.8. 
For  this  reason,  another  two  backup  rolls  are  used  to  minimize  bending  and  increase  the 
rigidity  of  the  system.  The  four  rolls  are  arranged  above  one  another  in  a  vertical  plane. 
Also,  the  backup  rolls  always  have  larger  diameters  than  those  of  the  working  rolls. 

Multihigh  rolling  mills  (Sendzimir  mills).  Multihigh  rolling  mills  are  used  particularly 
in  the  manufacture  of  very  thin  sheets,  those  with  a  thickness  down  to  0.0005  inch 
(0.01  mm)  and  a  width  up  to  80  inches  (2000  mm),  into  coils.  In  this  case,  the  work- 
ing rolls  must  have  very  small  diameters  (to  reduce  load  and  power  consumption,  as 
explained  before),  usually  in  the  range  of  3/8  inch  (10  mm)  up  to  1.25  inches  (30  mm). 


FIGURE    5.7 

The  five  main  types  of 
rolling  mills:  (a)  two- 
high  rolling  mill;  (b) 
three-high  rolling  mill; 

(c)  four-high  rolling  mill; 

(d)  multihigh  rolling  mill; 

(e)  universal  rolling  mill 


^ 


(a) 


(0 


3!^ — ^y}. 


(d) 


(e) 


5.2  Rolling  151 


FIGURE     5.8  Original  Distorted 

shape  roll 

..— y— -y_. 


Rolling  thin  sheets  with 
small-diameter  rolls 


Cross  section  of 
the  sheet 


Such  small-diameter  working  rolls  make  a  drive  practically  impossible.  They  are, 
therefore,  driven  by  friction  through  an  intermediate  row  of  driving  rolls  that  are,  in 
turn,  supported  by  a  row  of  backup  rolls.  This  arrangement  involves  a  cluster  of  either 
12  or  20  rolls,  resulting  in  exceptional  rigidity  of  the  whole  roll  system  and  almost 
complete  absence  of  working-roll  deflections.  An  equivalent  system  that  is  sometimes 
used  is  the  planetary  rolling  mill,  in  which  a  group  of  small-diameter  working  rolls  ro- 
tate around  a  large,  idle  supporting  roll  on  each  side  of  the  work. 

Universal  rolling  mills.  Universal  rolling  mills  are  used  for  producing  blooms  from  in- 
gots and  for  rolling  wide-flange  H  beams  (Gray's  beams).  In  this  type  of  mill,  there  are 
vertical  rolls  in  addition  to  the  horizontal  ones.  The  vertical  rolls  of  universal  mills  (for 
producing  structural  shapes)  are  idle  and  are  arranged  between  the  bearing  chocks  of 
the  horizontal  rolls  in  the  vertical  plane. 

The  Range  of  Rolled  Products 

The  range  of  rolled  products  is  standardized  in  each  country  in  the  sense  that  the 
shape,  dimensions,  tolerances,  properties,  and  the  like  are  given  in  a  standard  specifi- 
cations handbook  that  differs  from  country  to  country.  The  whole  range  of  rolled  prod- 
ucts can  generally  be  divided  into  the  following  four  groups. 

Structural  shapes  or  sections.  The  first  group  includes  general-purpose  sections  like 
round  and  square  bars;  angles;  channel,  H,  and  I  beams;  and  special  sections  (with  in- 
tricate shapes)  like  rails  and  special  shapes  used  in  construction  work  and  industry. 
Figure  5.9  shows  a  variety  of  sections  that  belong  to  this  group.  These  products  are 
rolled  in  either  rail  mills  or  section  mills,  where  the  body  of  each  roll  has  grooves 
called  passes  that  are  made  in  the  bodies  of  the  upper  and  lower  rolls  in  such  a  man- 
ner as  to  lie  in  the  same  vertical  plane.  They  are  used  to  impart  the  required  shape  to 
the  work.  This  process  is  carried  out  gradually  (i.e.,  the  stock  is  partly  deformed  at 
each  stand,  or  pass,  in  succession).  The  skill  of  a  rolling  engineer  is  to  plan  and  con- 
struct the  details  of  a  system  of  successive  passes  that  ensures  the  adequate  rolling  of 
blanks  into  the  desired  shape.  This  operation  is  called  roll  pass  design.  Figure  5.10a  il- 
lustrates the  roll  passes  for  producing  rails;  Figure  5.10b,  those  for  producing  an  I 
beam. 


152 


5      Metal  Forming 


FIGURE    5.9 

Some  structural  shapes 
or  sections  produced  by 
rolling 


1 


^ 


E^^ 


Square 


Slab 


Hexagonal 


Round 


/ 

bzzzzzzi 


Equal-sided  angle 


TZZZ7ZZD 

T 


>^ 


L-section 


I 


^sss 


SSSSSi 


I  beam 


VZZJVZZ2 


T-section 


/^»77>  Channel  beam  ^777^77* 


Rail 


Plates  and  sheets.  Plates  and  sheets  are  produced  in  plate  and  sheet  mills  for  the  hot 
rolling  of  metal  and  in  cold  reduction  mills  for  the  production  of  cold-rolled  coils, 
where  multihigh  rolling  mills  are  employed,  as  previously  mentioned.  This  group  of 
products  is  classified  according  to  thickness.  A  flat  product  with  a  width  ranging  from 
5/32  inch  (4  mm)  up  to  4  inches  (100  mm)  is  called  a  plate,  whereas  wider  and  thin- 
ner flat  stocks  are  called  sheets. 

Special-purpose  rolled  shapes.  This  group  includes  special  shapes,  one-piece  rolled 
wheels,  rings,  balls,  ribbed  tubes,  and  die-rolled  sections  in  which  the  cross  section 
of  the  bar  varies  periodically  along  its  length.  These  kinds  of  bars  are  used  in  the 
machine-building  industry  and  in  the  construction  industry  for  reinforcing  concrete 
beams  and  columns.  Figure  5.11a  shows  the  sequence  of  operations  in  manufacturing 
a  rolled  wheel  for  railway  cars;  Figure  5.11b,  the  wheel  during  the  final  stage  in  the 
rolling  mill. 

Seamless  tubes.    The  process  of  manufacturing  seamless  tubes  involves  two  steps: 

1.  Piercing  an  ingot  or  a  roughened-down  round  blank  to  form  a  thick-walled  shell 

2.  Rolling  the  obtained  shell  into  a  hollow  thin-walled  tube  having  the  desired  diam- 
eter and  wall  thickness 

In  the  first  step,  the  solid  blank  is  center-drilled  at  one  end,  heated  to  the  appropriate 
temperature,  and  then  placed  in  the  piercing  mill  and  forced  into  contact  with  the 
working  rolls.  There  are  several  types  of  piercing  mills,  but  the  commonly  used  one 
has  barrel-shaped  rolls.  As  Figure  5.12  shows,  the  axes  of  the  two  rolls  are  skew  lines, 
each  deviating  with  a  small  angle  from  the  direction  of  the  blank  axis.  Also,  the  two 
rol)s  rotate  in  the  same  direction,  forcing  the  blank  to  rotate  and  proceed  against  a 
mandrel.  A  hole  is  formed  and  becomes  larger;  finally,  a  rough  tube  is  obtained.  The 
milling  stand  is  provided  with  side  rollers  for  guiding  the  blank  and  the  formed  rough 
tube  during  this  operation.  In  the  second  step,  the  hollow  shell  (rough  tube)  is  usually 
forced  over  another  mandrel,  and  the  combination  is  longitudinally  rolled  at  their  hot 


5.2  Rolling 


153 


FIGURE    5.10 

Roll  passes:  (a)  for  producing  rails;  (b)  for  producing  an  I  beam 


Final 
pass 


i~ 


A  few 
sizing 
passes 


THT 


(b) 


Final 
pass 


state  between  grooved  rolls.  Mills  of  different  types  are  used,  including  continuous, 
automatic,  and  pilger  mills.  Finally,  a  sizing  operation  may  be  performed,  between  siz- 
ing rolls  and  without  the  use  of  a  mandrel,  at  room  temperature  in  order  to  improve  the 
properties  and  finish  of  the  tubes. 


Lubrication  in  Rolling  Processes 

Friction  plays  a  very  important  role  in  a  rolling  process  and  has  some  beneficial  ef- 
fects, provided  that  it  is  not  excessive.  In  fact,  it  is  responsible  for  shifting  the  work 
between  the  rolls  and  should  not,  therefore,  be  eliminated  or  reduced  below  an  appro- 
priate level.  This  is  an  important  point  to  be  taken  into  account  when  choosing  a  lu- 
bricant for  a  rolling  process. 


154 


Metal  Forming 


FIGURE    5.11 

The  production  of  a 
railway  car  wheel:  (a) 
sequence  of  stages;  (b) 
wheel  in  final  stage  in 
mill 


incj^ 


(b) 


In  the  cold  rolling  of  steel,  fluid  lubricants  of  low  viscosity  are  employed,  but 
paraffin  is  suitable  for  nonferrous  materials  like  aluminum  or  copper  alloys  to  avoid 
staining  during  subsequent  heat  treatment.  On  the  other  hand,  hot  rolling  is  often  car- 
ried out  without  lubricants  but  with  a  flood  of  water  to  generate  steam  and  break  up 
the  scales  that  are  formed.  Sometimes,  an  emulsion  of  graphite  or  graphited  grease  is 
used. 

Defects  in  Rolled  Products 

A  variety  of  defects  in  the  products  arise  during  rolling  processes.  A  particular  defect 
is  usually  associated  with  a  particular  process  and  does  not  arise  in  other  processes. 
Following  are  some  of  the  common  defects  in  rolled  products. 

Edge  cracking.  Edge  cracking  occurs  in  rolled  ingots,  slabs,  or  plates  and  is  believed 
to  be  caused  by  either  limited  ductility  of  the  work  metal  or  uneven  deformation,  es- 
pecially at  the  edges. 


FIGURE    5.12 

The  production  of 
seamless  tubes  by 
rolling 


5.3  Metal  Drawing  155 


FIGURE    5.13 

Alligatoring  when  rolling 

aluminum  slabs 

Arc  of       \                  /  \ 

contact  ~~~-V-^^  ^N- — \ 

mm 

Alligatoring.  Figure  5.13  shows  the  defect  of  alligatoring,  which  is  less  common 
than  it  used  to  be.  It  usually  occurs  in  the  rolling  of  slabs  (particularly  aluminum  al- 
loys), where  the  workpiece  splits  along  a  horizontal  plane  on  exit,  with  the  top  and 
bottom  parts  following  the  rotation  of  their  respective  rolls.  This  defect  always  oc- 
curs when  the  ratio  of  slab  thickness  to  the  length  of  contact  falls  within  the  range 
1.4  to  1.7. 

Folds.  Folds  are  defects  occurring  during  plate  rolling  when  the  reduction  per  pass  is 
too  small. 

Laminations.  Laminations  associated  with  cracking  may  develop  when  the  reduction 
in  thickness  is  excessive. 


.3  METAL  DRAWING 


Drawing  is  basically  a  forming  process  that  involves  pulling  a  slender  semifinished 
product  (like  wire,  bar  stock,  or  tube)  through  a  hole  of  a  drawing  die.  The  dimen- 
sions of  that  hole  are  smaller  than  the  dimensions  of  the  original  material.  Metals 
are  usually  drawn  in  their  cold  state,  and  the  required  shape  may  be  achieved  in  a 
single  drawing  operation  or  through  several  successive  drawing  operations,  in  which 
case  the  diameters  of  the  holes  are  successively  decreasing.  Sometimes,  annealing 
is  carried  out  between  the  drawing  operations  to  relieve  the  metal  from  work- 
hardening.  Accurate  dimensions,  good  surface  quality,  increased  strength  and  hard- 
ness, and  the  possibility  of  producing  very  small  sections  are  some  advantages  of 
the  drawing  process.  The  drawing  process  has,  therefore,  wide  industrial  application 
and  is  used  for  manufacturing  thin  wires,  thin-walled  tubes,  and  components  with 
sections  that  cannot  be  made  except  by  machining.  It  is  also  used  for  sizing  hot- 
rolled  sections. 


156 


Metal  Forming 


Preparing  the  Metal  for  Drawing 

Before  being  subjected  to  the  drawing  process,  metal  blanks  (wires,  rods,  or  tubes)  are 
heat  treated  and  then  cleaned  of  scales  that  result  from  that  operation.  Descaling  is  usu- 
ally done  by  pickling  the  heat-treated  metal  in  acid  solutions.  Steels  are  pickled  in  ei- 
ther sulfuric  or  hydrochloric  acid  or  a  mixture  of  both;  copper  and  brass  blanks  are 
treated  in  sulfuric  acid,  whereas  nickel  and  its  alloys  are  cleaned  in  a  mixture  of  sul- 
furic acid  and  potassium  bichromate.  After  pickling,  the  metal  is  washed  to  remove 
any  traces  of  acid  or  slag  from  its  surface.  The  final  operation  before  drawing  is  dry- 
ing the  washed  blanks  at  a  temperature  above  212°F  (100°C).  This  eliminates  the 
moisture  and  a  great  deal  of  the  hydrogen  dissolved  in  the  metal,  thus  helping  to  avoid 
pickling  brittleness. 

If  steel  is  to  be  subjected  to  several  successive  drawing  passes,  its  surface  should 
then  be  conditioned  for  receiving  and  retaining  the  drawing  lubricant.  Conditioning  is 
performed  directly  after  pickling  and  can  take  the  form  of  sulling,  coppering,  phos- 
phating,  or  liming.  In  sulling,  the  steel  rod  is  given  a  thin  coat  of  iron  hydroxide,  which 
combines  with  lime  and  serves  as  a  carrier  for  the  lubricant.  Phosphating  involves  ap- 
plying a  film  of  iron,  manganese,  or  zinc  phosphates  to  which  lubricants  stick  very 
well.  Liming  neutralizes  the  remaining  acid  and  forms  a  vehicle  for  the  lubricant.  Cop- 
pering is  used  for  severe  conditions  and  is  achieved  by  immersing  the  steel  rods  (or 
wires)  in  a  solution  of  vitriol.  All  conditioning  operations  are  followed  by  drying  at  a 
temperature  of  about  650°F  (300°C)  in  special  chambers. 

Wire  Drawing 

Drawing  dies.  A  die  is  a  common  term  for  two  parts:  the  die  body  and  the  die  holder. 
Die  bodies  are  made  of  cemented  carbides  or  hardened  tool  steel,  whereas  die  holders 
are  made  of  good-quality  tool  steel  that  possesses  high  toughness.  The  constructional 
details  of  a  die  are  shown  in  Figure  5.14.  It  can  be  seen  from  the  figure  that  the  die 
opening  involves  four  zones:  entry,  working  zone,  die  bearing,  and  exit.  The  entry  zone 
allows  the  lubricant  to  reach  the  working  zone  easily  and  also  protects  the  wire  (or  rod) 
against  scoring  by  sharp  edges.  The  working  zone  is  conical  in  shape  and  has  an  apex 


FIGURE    5.14 

The  constructional 
details  of  a  drawing  die 


Q *~  Drawing  force 


5.3  Metal  Drawing 


157 


angle  that  ranges  between  6°  and  24°,  depending  upon  the  type  of  work  and  the  metal 
being  drawn.  The  die  bearing,  sometimes  called  the  land,  is  a  short  cylindrical  zone  in 
which  a  sizing  operation  is  performed  to  ensure  accuracy  of  the  shape  and  dimensions 
of  the  end  product.  The  exit  zone  provides  back  relief  to  avoid  scoring  of  the  drawn 
wire  (or  rod).  In  a  wire-drawing  operation,  the  end  of  the  wire  is  pointed  by  swaging 
and  then  fed  freely  into  the  die  hole  so  that  it  appears  behind  the  die.  This  pointed  end 
is  gripped  by  the  jaws  of  a  carriage  that  pull  the  wire  through  the  die  opening,  where 
it  undergoes  reduction  in  cross-sectional  area  and  elongation  in  length. 

Draw  benches.  A  wire-drawing  operation  usually  involves  the  use  of  multidie  draw 
benches,  where  the  wire  passes  through  a  series  of  draw  plates.  First,  the  wire  leaves  the 
coil  and  passes  through  the  first  drawing  die.  Then,  it  is  wound  two  or  three  turns  around 
a  capstan  (drum)  before  it  enters  the  next  drawing  die.  A  typical  draw  bench  of  this  type 
with  six  draw  plates  is  shown  in  Figure  5.15.  In  practice,  a  bench  may  include  from  2  up 
to  22  draw  plates,  and  the  wire  leaving  the  last  die  may  attain  a  velocity  of  9800  feet  per 
minute  (50  m/s).  The  capstan  drives  are  designed  to  provide  not  only  forward  pull  after 
each  pass  but  also  backward  pull  to  the  wire  before  it  enters  the  next  drawing  die. 

Lubrication.  Lubrication  reduces  the  required  drawing  force  and  the  energy  con- 
sumed during  the  process,  increases  the  service  life  of  the  die,  and  allows  a  smoother 
wire  surface  to  be  obtained.  Various  kinds  of  soap  are  used  as  lubricants  in  wire- 
drawing processes.  Examples  are  sodium  soap  or  calcium  stearate,  which  is  picked  up 
by  the  wire  from  a  soap  box  adjacent  to  the  die.  Although  they  are  difficult  to  apply 
and  remove,  polymers  are  also  used  as  solid  lubricants,  especially  in  severe  conditions, 
as  in  the  case  of  drawing  hard  alloys  or  titanium.  Various  kinds  of  mineral  and  veg- 
etable oils  containing  fatty  or  chlorinated  additives  are  also  used  as  drawing  lubricants. 

Mechanics  of  wire  drawing.  The  state  of  stress  during  the  wire-drawing  process  (see 
Figure  5.14)  involves  compressive  forces  along  two  of  the  directions  and  tension  along 
the  third  one.  An  approximate  but  simple  estimate  of  the  drawing  force  can  be  given 
by  the  following  equation: 


F  =  a{xYx£n(^ 


(5.9) 


FIGURE    5.15 

A  typical  multidie  draw 
bench 


Draw  plates  (dies) 


Two  or  three 
j    capstan  drums 
and  draw  plates 


Original 
wire  coil 


Capstan 
drum 


158  5      Metal  Forming 


where:  a0  is  the  original  area 
af  is  the  final  area 
Y  is  the  mean  yield  stress  of  the  metal 

In  Equation  5.9,  the  ratio  ajaf  is  called  the  coefficient  of  elongation,  or  simply  the 
drawing  ratio.  In  industrial  practice,  it  is  usually  about  1.25  up  to  1.3.  Another  conju- 
gate term  that  is  used  in  drawing  processes  is  the  reduction,  given  by  the  following 
equation: 

reduction  r  =  a°~a{  x  100  (5.10) 

a0 

The  theoretically  obtained  maximum  value  for  the  reduction  is  64  percent;  however,  it 
usually  does  not  exceed  about  40  percent  in  industry. 

Defects  in  wire  drawing.  Structural  damage  in  the  form  of  voids  or  cracks  occurs  in 
different  forms  in  wire-drawing  processes  under  certain  conditions.  Following  are 
some  of  the  defects  encountered: 

1.  Internal  bursts  in  wire,  taking  the  form  of  repeating  internal  cup  and  cone  frac- 
tures (cuppy  wire),  usually  occur  when  drawing  heavily  cold-worked  copper 
under  conditions  of  light  draft  and  very  large  die  angles. 

2.  Similar  centerline  arrowhead  fractures  occur  if  the  blank  is  a  sheet  and  when  the 
die  angle  and  reduction  produce  severe  tension  on  the  centerline. 

3.  Transverse  surface  cracking  may  occur  as  a  result  of  longitudinal  tension  stresses 
in  the  surface  layers. 

Tube  Drawing 

Diameter  and  thickness  of  pipes  can  be  reduced  by  drawing.  Figure  5.16  illustrates  the 
simplest  type  of  tube  drawing.  The  final  tube  thickness  is  affected  by  two  contradict- 
ing factors.  The  longitudinal  stress  tends  to  make  the  wall  thinner,  whereas  the  cir- 
cumferential stress  thickens  it.  If  a  large  die  angle  is  used,  the  thinning  effect  will 
dominate. 

The  technique  shown  in  Figure  5.17  of  using  a  fixed  plug  reduces  the  tube  diam- 
eter and  controls  its  thickness.  However,  a  disadvantage  of  this  type  of  tube  drawing  is 
the  limitation  imposed  on  the  length  of  the  tube  by  the  length  of  the  mandrel.  When 
tubes  having  longer  length  are  to  be  drawn,  a  floating  mandrel  like  that  shown  in  Fig- 
ure 5.18  is  then  employed.  Another  method  that  has  gained  widespread  application  is 
using  a  removable  mandrel  like  that  shown  in  Figure  5.19. 


5.4  EXTRUSION 


Extrusion  involves  forcing  a  billet  that  is  enclosed  in  a  container  through  an  open- 
ing whose  cross-sectional  area  and  dimensions  are  smaller  than  those  of  the  original 
billet.  The  cross  section  of  the  extruded  metal  will  conform  to  that  of  the  die  open- 


FIGURE    5.16 

Simplest  type  of  tube 
drawing 


FIGURE    5.17 

Tube  drawing  using  a 
fixed  plug 


- —       Pull 


FIGURE    5.18 

Tube  drawing  using  a 
floating  mandrel 


\\\V\V\\\\V- 


^^■w 


Pull 


ing.  Historically,  extrusion  was  first  used  toward  the  end  of  the  eighteenth  century  for 
producing  lead  pipes.  It  later  gained  widespread  industrial  application  for  processing 
nonferrous  metals  and  alloys  like  copper,  brass,  aluminum,  zinc,  and  magnesium.  Re- 
cently, with  the  modern  developments  in  extrusion  techniques,  lubricants,  and  tool- 
ing, other  metals,  such  as  steels,  titanium,  refractory  metals,  uranium,  and  thorium, 
can  also  be  extruded  successfully.  The  stock  used  for  extrusion  is  mainly  a  cast  ingot 
or  a  rolled  billet.  Any  surface  defects  in  the  original  billets  must  be  removed  by  saw- 
ing, shearing,  turning,  or  any  other  appropriate  machining  operation  before  the  ex- 


160 


Metal  Forming 


FIGURE    5.19 

Tube  drawing  using  a 
removable  mandrel 


trusion  process  is  performed.  Extrusion  carried  out  when  the  billets  are  at  their  cold 
state  is  known  as  cold  extrusion;  when  they  are  at  elevated  temperatures,  it  is  known 
as  hot  extrusion.  In  this  latter  case,  the  container,  the  die,  and  the  pressing  plunger 
must  be  heated  to  a  temperature  of  about  650°F  (350°C)  prior  to  each  extrusion 
cycle. 


Types  of  Extrusion 

Direct  extrusion.  Direct  extrusion  is  used  in  the  manufacture  of  solid  and  hollow 
slender  products  and  for  structural  shapes  that  cannot  be  obtained  by  any  other  metal 
forming  process.  Figure  5.20  illustrates  the  working  principles  of  this  method,  and  Fig- 
ure 5.21  shows  the  details  of  an  extrusion  die  arrangement  for  producing  channel  sec- 
tions. As  can  be  seen,  during  an  extrusion  process  a  billet  is  pushed  out  of  the  die  by 
a  plunger  and  then  slides  along  the  walls  of  the  container  as  the  operation  proceeds.  At 
the  end  of  the  stroke,  a  small  piece  of  metal  (stub-end  scrap)  remains  unextruded  in  the 
container. 


FIGURE    5.20 

Principles  of  direct 
extrusion  for  producing 
solid  objects 


Plunger 


Container 


5.4  Extrusion 


161 


The  extruded  product  is  separated  by  shearing,  and  the  stub-end  is  then  ejected  out 
of  the  container  after  the  plunger  is  withdrawn.  Also,  the  leading  end  of  the  extruded 
product  does  not  undergo  enough  deformation.  It  is,  therefore,  poorly  shaped  and  must 
be  removed  as  well.  Obviously,  the  efficiency  of  material  utilization  in  this  case  is  low, 
and  the  waste  can  amount  to  10  or  even  15  percent,  as  opposed  to  rolling,  where  the 
waste  is  only  1  to  3  percent.  This  makes  the  productivity  of  direct  extrusion  quite  in- 
ferior to  that  of  rolling. 


FIGURE    5.21 

Typical  extrusion  die 
arrangement  for 
producing  channel 
sections 


Container 


162 


5      Metal  Forming 


FIGURE    5.22 

Direct  extrusion  for 
producing  hollow 
objects 


Figure  5.22  illustrates  the  technique  used  for  producing  hollow  sections  and  tubes. 
As  can  be  seen,  a  mandrel  or  a  needle  passes  freely  through  a  hole  in  the  blank  and  the 
die  opening.  If  the  die  opening  is  circular,  an  annular  clearance  between  the  die  open- 
ing and  the  mandrel  results.  When  the  metal  is  extruded  through  the  annular  clearance, 
it  forms  a  tube.  A  hole  has  to  be  pierced  or  drilled  into  the  original  blank  before  it  is 
extruded. 

Based  on  this  discussion,  it  is  clear  that  the  conventional  extrusion  process  has  the 
advantages  of  high-dimensional  accuracy  and  the  possibility  of  producing  complex 
sections  from  materials  having  poor  plasticity.  On  the  other  hand,  its  disadvantages  in- 
clude low  productivity,  short  tool  life,  and  expensive  tooling.  Therefore,  the  process  is 
usually  employed  for  the  manufacture  of  complex  shapes  with  high-dimensional  accu- 
racy, especially  when  the  material  of  the  product  has  a  low  plasticity.  Figures  5.23  and 
5.24  show  some  extruded  sections  and  parts,  and  Figure  5.25  shows  some  final  prod- 
ucts assembled  from  extruded  sections. 


FIGURE    5.23 

Some  extruded 
sections  (Courtesy  of 
Midwest  Aluminum, 
Inc.,  Kalamazoo, 
Michigan) 


5.4  Extrusion 


163 


FIGURE    5.24 

Some  extruded  parts 
(Courtesy  of  Midwest 
Aluminum,  Inc., 
Kalamazoo,  Michigan) 


0 


# 


• 


Indirect  extrusion.  In  indirect  extrusion,  the  extrusion  die  is  mounted  on  a  hollow 
ram  that  is  pushed  into  the  container.  Consequently,  the  die  applies  pressure  to  the  bil- 
let, which  undergoes  plastic  deformation.  As  shown  in  Figure  5.26,  the  metal  flows  out 
of  the  die  opening  in  a  direction  opposite  to  the  ram  motion.  There  is  almost  no  slid- 
ing motion  between  the  billet  and  the  container  walls.  This  eliminates  friction,  and  the 
extrusion  load  will  be  lower  than  that  required  in  forward  direct  extrusion  by  about  30 
percent.  Also,  the  amount  of  waste  scrap  is  reduced  to  only  5  percent.  Nevertheless,  in- 
direct extrusion  finds  only  limited  application  due  to  the  complexity  and  the  cost  of 
tooling  and  press  arrangement  required. 

Another  indirect  extrusion  method,  usually  called  backward  or  reverse  extrusion, 
used  in  manufacturing  hollow  sections  is  shown  in  Figure  5.27.  In  this  case,  the  metal 
is  extruded  through  the  gap  between  the  ram  and  the  container.  As  in  indirect  extrusion 
for  solid  objects,  the  ram  and  the  product  travel  in  opposite  directions. 


164 


5      Metal  Forming 


FIGURE    5.25 

Some  products  assembled  from  extruded  sections  (Courtesy  of  Midwest  Aluminum,  Inc. 
Kalamazoo,  Michigan) 


Hydrostatic  extrusion.  A  radical  development  that  eliminates  the  disadvantages  of 
cold  extrusion  (like  higher  loads)  involves  hydrostatic  extrusion.  Figure  5.28  illus- 
trates the  basic  principles  of  this  process,  where  the  billet  is  shaped  to  fit  the  die  and 
surrounded  by  a  high-pressure  hydraulic  fluid  in  a  container.  When  the  plunger  is 
pressed,  it  increases  the  pressure  inside  the  container,  and  the  resulting  high  pressure 
forces  the  billet  to  flow  through  the  die.  Friction  between  the  billet  and  the  container 
is  thus  eliminated,  whereas  friction  between  the  billet  and  the  die  is  markedly  re- 


FIGURE    5.26 

Indirect  extrusion  for 
producing  solid  objects 


5.4  Extrusion 


165 


FIGURE    5.27 

Indirect  (backward) 
extrusion  for  producing 
hollow  objects 


FIGURE    5.28 

Principles  of 
hydrostatic  extrusion 


Billet 


Fluid 


duced.  Also,  the  buckling  effect  of  longer  billets  is  eliminated  because  virtually  the 
entire  length  of  the  billet  is  subjected  to  hydrostatic  pressure.  This  makes  it  possible 
to  extrude  very  long  billets. 

Impact  extrusion.  Impact  extrusion  involves  striking  a  cold  slug  of  soft  metal  (like 
aluminum)  that  is  held  in  a  shallow  die  cavity  with  a  rapidly  moving  punch,  thus  caus- 
ing the  metal  to  flow  plastically  around  the  punch  or  through  the  die  opening.  The  slug 
itself  is  a  closely  controlled  volume  of  metal  that  is  lubricated  and  located  in  the  die 
cavity.  The  press  is  then  activated,  and  the  high-speed  punch  strikes  the  slug.  A  fin- 
ished impacted  product  is  extruded  with  each  stroke  of  the  press.  These  products  are 
not  necessarily  cylindrical  with  a  circular  cross  section.  In  fact,  the  range  of  shapes 
possible  is  very  broad,  including  even  irregular  symmetrical  shapes,  as  shown  in  Fig- 
ures 5.29  and  5.30.  There  are  three  types  of  the  impact  extrusion  processes:  forward, 
reverse,  and  combination  (the  names  referring  to  the  direction  of  motion  of  the  de- 
forming metal  relative  to  that  of  the  punch). 

Figure  5.31  illustrates  the  basic  principles  of  reverse  impact  extrusion.  It  is  used 
for  manufacturing  hollow  parts  with  forged  bases  and  extruded  sidewalls.  The  flowing 
metal  is  guided  only  initially;  thereafter,  it  goes  by  its  own  inertia.  This  results  in  the 
elimination  of  friction  and,  therefore,  an  appreciable  reduction  in  the  load  and  energy 
required.  A  further  advantage  is  the  possibility  of  producing  thinner  walls. 

The  principles  of  forward  impact  extrusion  are  illustrated  in  Figure  5.32.  It  is 
mainly  employed  in  producing  hollow  or  semihollow  products  with  heavy  flanges  and 
multiple  diameters  formed  on  the  inside  and  outside.  Closer  wall  tolerances,  larger 
slenderness  ratios,  better  concentricities,  and  sound  thinner  sections  are  among  the  ad- 
vantages of  this  process. 


166 


5      Metal  Forming 


FIGURE    5.29 

Some  shapes  produced 
by  impact  extrusion 
(Courtesy  of  Metal 
Impact  Corporation, 
Rosemont,  Illinois) 


Complex  shapes  can  be  produced  by  a  combination  of  the  two  preceding 
processes,  which  are  performed  simultaneously  in  the  same  single  stroke,  as  shown  in 
Figure  5.33.  Like  the  other  impact  extrusion  methods,  this  process  has  the  advantage 
of  cleaner  product  surfaces,  elimination  of  trimming  or  further  machining  operations, 
and  higher  strength  of  the  parts  obtained. 

Mechanics  of  Extrusion 

We  can  clearly  see  (from  Figure  5.20)  that  an  element  of  the  deforming  metal  being 
extruded  is  subjected  to  a  state  of  stress  involving  triaxial  compression.  This  all-around 
high  pressure  results  in  a  marked  improvement  in  the  plasticity  of  the  metal.  Conse- 
quently, extrusion  can  be  employed  when  working  metal  having  poor  plasticity,  as  op- 
posed to  rolling  or  wire  drawing,  where  only  ductile  metals  can  be  formed  (worked). 


5.4  Extrusion 


167 


FIGURE    5.30 

Some  components 
produced  by  impacting 
(Courtesy  of  Metal 
Impact  Corporation, 
Rosemont,  Illinois) 


Load  requirement.  For  the  sake  of  simplicity,  it  is  sometimes  assumed  that  the 
processes  involve  ideal  deformation  without  any  friction.  The  extrusion  pressure  can 
then  be  given  by  the  following  equation: 


Pextrus.on  =  Y  X  €n^  =YX  (,lR 


(5.11) 


FIGURE    5.31 

Principles  of  reverse 
impact  extrusion 


PQ 


& 


168 


5      Metal  Forming 


FIGURE    5.32 

Principles  of  forward 
impact  extrusion 


FIGURE    5.33 

Combination  impacting 


T 


Y/ 

1 

I 

y, 

// 

V, 

where:  aQ  is  the  original  cross-sectional  area 

af  is  the  final  cross-sectional  area  after  extrusion 

R  is  the  extrusion  ratio 

Y  is  the  mean  yield  stress  of  the  metal 

The  extrusion  load  is,  therefore, 

F  —  p  x  a0 


(5.12) 


These  equations  are  used  to  give  only  rough  estimates  because  actual  extrusion 
processes  involve  friction  and  the  lack  of  homogeneous  deformation  of  the  metal,  as 
will  be  seen  later.  Therefore,  research  workers  developed  several  empirical  formulas  to 
give  the  extrusion  pressure  as  a  function  of  the  extrusion  ratio  and  the  mechanical 
properties  of  the  metal.  A  convenient  formula  was  proposed  by  W.  Johnson  (the  emi- 
nent British  researcher  in  the  area  of  metal  forming)  as  follows: 


=  Y  0.8+  l.5€n 


1 


1  -r 


(5.13) 


5.4  Extrusion 


169 


In  Equation  5.13,  r  is  the  reduction  given  by 
reduction  r  =  — ^ — - 


(5.14) 


Metal  flow  and  deformation.  To  study  metal  flow,  let  us  consider  extruding  a  split  bil- 
let involving  two  identical  halves,  with  a  rectangular  grid  engraved  on  the  meridional 
plane  of  each  half.  The  separation  surface  is  covered  with  lanolin  or  a  similar  appro- 
priate material  to  prevent  welding  or  sticking  of  the  two  halves  during  the  process. 
After  extruding  the  split  billet,  the  two  halves  are  separated,  and  the  distortion  of  the 
grid  can  be  investigated.  Figure  5.34  shows  the  grid  after  extrusion.  We  can  see  that 
the  units  of  the  grid,  which  were  originally  square  in  shape,  became  parallelograms, 
trapezoids,  and  other  shapes.  The  following  can  also  be  observed: 

1.  The  velocity  of  the  core  is  greater  than  that  of  the  outer  layers. 

2.  The  outer  layers  are  deformed  to  a  larger  degree  than  the  core. 

3.  The  leading  end  of  the  extruded  part  is  almost  undeformed. 

4.  The  metal  adjacent  to  the  die  does  not  flow  easily,  leading  to  the  initiation  of 
zones  where  little  deformation  occurs.  These  zones  are  called  dead-metal  zones. 

In  fact,  the  preceding  method  for  studying  the  metal  flow  is  usually  used  with  models 
made  of  wax,  plasticine,  and  lead  to  predict  any  defect  that  may  occur  during  the  ac- 
tual process  so  that  appropriate  precautions  can  be  taken  in  advance. 

Lubrication  in  Extrusion 

Friction  at  the  billet-die  and  billet-container  interfaces  increases  the  load  and  the 
power  requirement  and  reduces  the  service  life  of  the  tooling.  For  these  reasons,  lubri- 
cants are  applied  to  the  die  and  container  walls. 

As  in  wire  drawing,  soaps  and  various  oils  containing  chlorinated  additives  or 
graphite  are  used  as  lubricants  in  cold  extrusion  of  most  metals,  whereas  lanolin  is  usu- 
ally used  for  the  softer  ones.  For  hot  extrusion  of  mild  steel,  graphite  is  an  adequate  lu- 
bricant. It  is  not,  however,  recommended  for  high-temperature  extrusions,  such  as 
extruding  molybdenum  at  3250°F  (1800°C);  in  this  case,  glass  is  the  most  successful 
lubricant. 

Defects  in  Extruded  Products 

Defects  in  extruded  parts  usually  fall  into  one  of  three  main  categories:  surface  or  in- 
ternal cracking,  sinking  (piping),  and  skin-inclusion  defects.  Cracking  is  caused  by 
secondary  tensile  stresses  acting  within  a  material  having  low  plasticity.  Cracking  can 
occur  on  the  surface  of  a  relatively  brittle  material  during  the  extrusion  process,  and  it 


FIGURE    5.34 

Distorted  grid  indicating 
metal  flow  in  extrusion 


170 


Metal  Forming 


may  also  occur  in  the  form  of  fire-tree  or  central  bursts  when  extruding  materials  like 
bismuth,  magnesium,  60/40  brass,  steel,  and  brittle  aluminum  alloys.  Piping  involves 
sinking  of  the  material  at  the  rear  of  the  stub-end.  This  defect  is  usually  encountered 
toward  the  end  of  the  extrusion  stroke,  especially  when  the  original  billets  are  rela- 
tively short.  Skin-inclusion  defects  may  take  different  forms,  depending  upon  the  de- 
gree of  lubrication  and  the  hardness  of  the  surface  layer  of  the  original  stock.  When 
extruding  lubricated  billets  of  high-copper  alloys,  the  surface  skin  will  slide  over  the 
container  wall  and  then  penetrate  the  billet,  as  illustrated  in  Figure  5.35,  where  the 
three  different  extrusion  defects  are  sketched. 


FIGURE    5.35 

Three  different  defects 
occurring  in  an 
extrusion  process 


A 


Central 
burst 


Piping 


Skin-inclusion 


Design  Considerations 

Conventional  extrusions.  When  making  parts  that  have  constant  cross  sections,  the 
extrusion  process  is  usually  more  economical  and  faster  than  machining,  casting,  or 
fabricating  the  shapes  by  welding  (or  riveting).  Also,  the  designer  of  the  extruded  sec- 
tion is  relatively  free  to  put  the  metal  where  he  or  she  wants.  Nevertheless,  there  are 
some  design  guidelines  that  must  be  taken  into  consideration  when  designing  an  ex- 
truded section: 

1.  The  circle  size  (i.e.,  the  diameter  of  the  smallest  circle  that  will  enclose  the  ex- 
trusion cross  section)  can  be  as  large  as  31  inches  (775  mm)  when  extruding  light 
metals. 

2.  Solid  shapes  are  the  easiest  to  extrude.  Semihollow  and  hollow  shapes  are  more 
difficult  to  extrude,  especially  if  they  have  thin  walls  or  include  abrupt  changes  in 
wall  thickness. 

3.  Wall  thicknesses  must  be  kept  uniform.  If  not,  all  transitions  must  be  streamlined 
by  generous  radii  at  the  thick-thin  junctions. 

4.  Sharp  corners  at  the  root  of  a  die  tongue  should  be  avoided  when  extruding  semi- 
hollow  sections. 

5.  A  complicated  section  should  be  broken  into  a  group  of  simpler  sections  that  are  as- 
sembled after  the  separate  extrusion  processes.  In  such  a  case,  the  sections  should 
be  designed  to  simplify  assembly;  for  example,  they  should  fit,  hook,  or  snap  to- 
gether. Screw  slots  or  slots  to  receive  other  tightening  material,  such  as  plastic,  may 
also  be  provided. 


5.4  Extrusion 


171 


Figure  5.36  illustrates  some  recommended  designs  for  assembling  extruded  alu- 
minum sections.  Figure  5.37  illustrates  and  summarizes  some  recommended  designs  as 
well  as  those  to  be  avoided  as  general  guidelines  for  beginning  designers. 

Aluminum  impact  extrusions.  In  order  to  accomplish  good  designs  of  aluminum  im- 
pact extrusions,  all  factors  associated  with  and  affecting  the  process  must  be  taken  into 
account.  Examples  are  alloy  selection,  tool  design,  lubrication,  and,  of  course,  the  gen- 
eral consideration  of  mechanical  design.  Following  are  some  basic  guidelines  and  de- 
sign examples: 

1.  Use  alloys  that  apply  in  the  desired  case  and  have  the  lowest  strength. 

2.  An  impact  extrusion  should  be  symmetrical  around  the  punch. 

3.  Threads,  cuts,  projections,  and  the  like  are  made  by  subjecting  the  impact  extru- 
sions to  further  processing. 

4.  For  reverse  extrusions,  the  ratio  of  maximum  length  to  internal  diameter  must  not 
exceed  8  to  avoid  failure  of  long  punches. 

5.  A  small  outer-corner  radius  must  be  provided  for  a  reverse  extrusion,  but  the 
inner-corner  radius  must  be  kept  as  small  as  possible  (see  Figure  5.38a). 

6.  The  thickness  of  the  bottom  near  the  wall  must  be  15  percent  greater  than  the 
thickness  of  the  wall  itself  to  prevent  shear  failure  (see  Figure  5.38b). 

7.  The  inside  bottom  should  not  be  completely  flat.  To  avoid  the  possibility  of  the  punch 
skidding  on  the  billet,  only  80  percent  of  it  at  most  can  be  flat  (see  Figure  5.38c). 


FIGURE    5.36 

Some  recommended 
designs  for  assembling 
extruded  aluminum 
sections  (Courtesy  of 
the  Aluminum 
Association,  Inc., 
Washington,  D.C.) 


Single 


Lap  joints 
H    2 


Double 


Held  by 

self  threading 

fastener 


Lap-lock  joints 
V\    5 


Side  entry       Edge  entry       Dovetail 
sliding  fit 


Cylindrical  sliding  fits 
H    8  9 


Fixed 


Adjustable     As  adapted  to 
stair  riser 


172 


Metal  Forming 


FIGURE    5.37 

Some  design 
considerations  for 
conventional  extrusions 
(Courtesy  of  the 
Aluminum  Association, 
Inc.,  Washington,  D.C.) 


Poor 


^^ 


\w\w 


kmmmvKas 


sssssssssgssgg 


Li 


^^    ^^ 


m\\s\\\s\\s\\\w 


JWs>    g^^i 


?//;w 


•-   c 


ggggssas^BssaassSfl 


Good 


fcmmmvMs^a 


^^    ^^ 


^mmvmw 


I 


I 


;^\VV\VVVVVVVV\W 


^^vvvv^\\\\v\v^ 


Reason 


SYMMETRY  PREFERRED  IN  SEMI-HOLLOW  AREAS 
When  designing,  visualize  the  die  and  tongue  that  will  be 
necessary  to  produce  a  semi-hollow  shape.  By  keeping 
the  void  symmetrical  you  lessen  the  chances  that  the  die 
tongue  may  break. 


ROUNDED  CORNER  STRENGTHENS  TONGUE 
The  preceding  cross  section  has  been  further  improved. 
The  die  tongue  is  now  less  likely  to  snap  off. 


REDUCE  AREA  OF  VOID-1 

Further  improvement  results  if  outline  can  be  changed  to 
reduce  area  enclosed.  Reduced  area  means  less  pressure 
on  the  tongue;  easier  extrusion. 


AVOID  HOLLOW  SHAPE 

Hollow  and  multi-hollow  extruded  shapes  are  usually 
much  more  costly  than  the  simple  solid  shape.  Also  less 
metal  has  been  used. 


WEB  GIVES  BETTER  DIMENSIONAL  CONTROL 

Metal  dimensions  are  more  easily  held  than  gap  or  angle 

dimensions.  Web  also  allows  thinner  wall  sections  in  this 

example. 

The  hollow  condition  of  the  "redesigned"  part  can  be 

avoided  by  making  the  component  in  two  pieces  as  shown 

by  the  dotted  line. 


SMOOTH  ALL  TRANSITIONS 

Transitions  should  be  streamlined  by  a  generous  radius 

at  any  thick-thin  junction. 


KEEP  WALL  THICKNESS  UNIFORM 

The  preceding  shape  can  be  further  improved  by 

maintaining  uniform  wall  thickness. 

In  addition  to  using  more  metal,  thick-thin  junctions 

giv  rise  to  distortion,  die  breakage  or  surface  defects 

on  the  extrusion. 


RIBS  HELP  STRAIGHTENING  OPERATION 
Wide,  thin  sections  can  be  hard  to  straighten  after 
extrusion.  Ribs  help  prevent  twisting. 


5.4  Extrusion 


173 


FIGURE    5.38 

Some  design 
considerations  for 
impact  extrusions:  (a) 
corner  radii  for  reverse 
extrusion;  (b)  thickness 
of  the  bottom  near  the 
wall;  (c)  inside  bottom; 
(d)  ribs;  (e)  multiple- 
diameter  parts 
(Courtesy  of  the 
Aluminum  Association, 
Inc.,  Washington,  D.C.) 


Preferably  small 
as  possible 


ftfr77777f 


Approx.       "^ 
15°  3_ 


Ar-Jf 


r-  as 
small  as 
*^  possible         /     J 

xgzzzzzzzzZSz 


w 


Bottom  =  1.151V 


[*—  0.8D-*] 


approx. 


approx. 
(c) 


32 


Good 


Poor 


(d) 


(e) 


8.  External  and  internal  bosses  are  permitted,  provided  that  they  are  coaxial  with  the 
part.  However,  the  diameter  of  the  internal  boss  should  not  be  more  than  1/4  of  the 
internal  diameter  of  the  shell. 

9.  Longitudinal  ribs,  whether  external  or  internal,  on  the  full  length  of  the  impact  ex- 
trusion are  permitted.  They  should  preferably  be  located  in  a  symmetrical  distrib- 
ution. However,  the  height  of  each  rib  must  not  exceed  double  the  thickness  of  the 
wall  of  the  shell  (see  Figure  5.38d).  The  main  function  of  ribs  is  to  provide  stiff- 
ness to  the  walls  of  shells.  They  are  also  sometimes  used  for  other  reasons,  such 
as  to  provide  locations  for  drilling  and  tapping  (to  assemble  the  part),  to  enhance 
cooling  by  radiation,  and  to  provide  an  appropriate  gripping  surface. 


174 


Metal  Forming 


10.  An  impact  extrusion  can  have  a  wall  with  varying  thickness  along  its  length  (i.e., 
it  can  be  a  multiple-diameter  part).  However,  internal  steps  near  the  top  of  the 
product  should  be  avoided  because  they  cause  excessive  loading  and  wear  of  the 
punch  (see  Figure  5.38e). 

11.  Remember  that  it  is  sometimes  impossible  to  obtain  the  desired  shape  directly  by 
impacting.  However,  an  impact  extrusion  can  be  considered  as  an  intermediate  prod- 
uct that  can  be  subjected  to  further  working  or  secondary  operations  like  machining, 
flange  upsetting,  nosing  and  necking,  or  ironing  (see  Figure  5.39a  through  c). 

Again,  in  addition  to  the  preceding  guidelines,  general  rules  of  mechanical  design 
as  well  as  common  engineering  sense  are  necessary  for  obtaining  a  successful  design 
for  the  desired  product.  It  would  also  be  beneficial  for  the  beginner  to  look  at  various 
designs  of  similar  parts  and  to  consult  with  experienced  people  before  starting  the  de- 
sign process.  Given  in  Figure  5.40  are  sketches  reflecting  good  design  practice  for 
some  impact-extruded  tubular  parts  and  shells. 


FIGURE    5.39 

Some  secondary 
operations  after  impact 
extruding:  (a)  flange 
upsetting;  (b)  nosing; 
(c)  ironing  (Courtesy  of 
the  Aluminum 
Association,  Inc., 
Washington,  D.C.) 


(b) 


(c) 


5.4  Extrusion 


175 


FIGURE    5.40 

Sketches  reflecting 
good  design  practice 
for  some  impact- 
extruded  tubular  parts 
and  shells  (Courtesy  of 
Aluminum  Association, 
Inc.,  Washington,  D.C.) 


Tubular  Parts 


<^yyyyyyyyyyyy^A^yyyyM^^^_ 


^v^^^^^^^Mw^^^^^^^^^^^^^^^^y 


Flanged  tube  with  open  end. 


vzzzzzzzzzzzzzz 


Cup  and  tube  assembly, 
extruded  as  single  piece. 


>/SSS/SSSSS/////////////77Z. 


'/ss;;;s>;;;;ss/////w/777; 


Flange  end  closed. 


W//////////////&& 


V////////////////J& 


Flange  with  multiple 
step-down  diameters. 


Flanged  tube  with 
multiple  diameters. 


Partially  closed  end  tube 
with  heavy  flange. 


rysss;ss;y>y's;/s;;;s>;s;;sss//sS;/;, 


^ssssssssssss//////;;/;. 


'///////////////S^z/A 


yyyyyyyyy/y/y/y///y/yA 


Combination  impact  with  the 
flange  at  the  midpoint.  Such  an 
impact  also  serves  as  a  transition 
from  one  diameter  to  another. 
Wall  thicknesses  can  also  be  varied. 


Shells 


Uniform  wall  thickness 
with  flanged  end  open. 


Outside  longitudinal  ribs  can  be 
spaced  equally  or  in  symmetrical 
patterns.  Ribs  may  be  extended 
to  become  cooling  fins. 


Short  recessed  ribs  in  bottom  can  be 
used  for  tool  insertions,  drive,  etc. 


>///>////>//>//*/?, 


An  external  boss  can  be  combined 
with  an  internal  center  tube. 


,',  1  J  >  >  >  1  >  1  >  >  /  S  J  J  /  l-7-TTs 

r-T-j  i  >/>>>!  ///>//  m 

An  integral  center  tube  can  be 
formed  so  that  assembly  and 
machining  are  not  needed. 


Inside  bosses  can  be  produced 
as  integral  parts  of  the  closed 
end.  The  side  wall  can  have 
longitudinal  internal  ribs. 


Combination  impact. 


176 


Metal  Forming 


ORGING 


The  term  forging  is  used  to  define  the  plastic  deformation  of  metals  at  elevated  tem- 
peratures into  predetermined  shapes  using  compressive  forces  that  are  exerted  through 
dies  by  means  of  a  hammer,  a  press,  or  an  upsetting  machine.  Like  other  metal  form- 
ing processes,  forging  refines  the  microstructure  of  the  metal,  eliminates  the  hidden  de- 
fects such  as  hair  cracks  and  voids,  and  rearranges  the  fibrous  macrostructure  to 
conform  with  the  metal  flow.  It  is  mainly  the  latter  factor  that  gives  forging  its  merits 
and  advantages  over  casting  and  machining.  By  successful  design  of  the  dies,  the 
metal  flow  during  the  process  can  be  employed  to  promote  the  alignment  of  the  fibers 
with  the  anticipated  direction  of  maximum  stress.  A  typical  example  is  shown  in  Fig- 
ure 5.41,  which  illustrates  the  fibrous  macrostructure  in  two  different  crankshafts  pro- 
duced by  machining  from  a  bar  stock  and  by  forging.  As  can  be  seen,  the  direction 
of  the  fibers  in  the  second  case  is  more  favorable  because  the  stresses  in  the  webs 
when  the  crankshaft  is  in  service  coincide  with  the  direction  of  fibers  where  the 
strength  is  maximum. 

A  large  variety  of  materials  can  be  worked  by  forging.  These  include  low-carbon 
steels,  aluminum,  magnesium,  and  copper  alloys,  as  well  as  many  of  the  alloy  steels 
and  stainless  steels.  Each  metal  or  alloy  has  its  own  plastic  forging  temperature  range. 
Some  alloys  can  be  forged  in  a  wide  temperature  range,  whereas  others  have  narrow 
ranges,  depending  upon  the  constituents  and  the  chemical  composition.  Usually,  the 
forging  temperatures  recommended  for  nonferrous  alloys  and  metals  are  much  lower 
than  those  required  for  ferrous  materials.  Table  5.2  indicates  the  range  of  forging  tem- 
peratures for  the  commonly  used  alloys. 

Forged  parts  vary  widely  in  size  ranging  from  a  few  pounds  (less  than  a  kilogram) 
up  to  300  tons  (3  MN)  and  can  be  classified  into  small,  medium,  and  heavy  forgings. 


FIGURE    5.41 

The  fibrous 
macrostructure  in  two 
crankshafts  produced 
by  machining  and  by 
forging 


Produced  by  machining 
from  a  bar  stock 


Produced  by  forging 


TABLE    5.2 

Forging  temperature 
range  for  different 
metals 


Metal 

Forging  Temperature 

Low-carbon  steel 

1450-2550°F  (800-1400°C) 

Aluminum 

645-900°F    (340-480°C) 

Magnesium 

645-800°F    (340-430°C) 

Copper 

800-1900°F  (430-1040°C) 

Brass 

1100-1700°F (590-930°C) 

5.5  Forging  177 

Small  forgings  are  illustrated  by  small  tools  such  as  chisels  and  tools  used  in  cutting 
and  carving  wood.  Medium  forgings  include  railway-car  axles,  connecting  rods,  small 
crankshafts,  levers,  and  hooks.  Among  the  heavier  forgings  are  shafts  of  power-plant 
generators,  turbines,  and  ships,  as  well  as  columns  of  presses  and  rolls  for  rolling 
mills.  Small  and  medium  forgings  are  forged  from  rolled  sections  (bar  stocks  and 
slabs)  and  blooms,  whereas  heavier  parts  are  worked  from  ingots. 

All  forging  processes  fall  under  two  main  types:  open-die  forging  processes,  in 
which  the  metal  is  worked  between  two  flat  dies,  and  closed-die  forging  processes,  in 
which  the  metal  is  formed  while  being  confined  in  a  closed  impression  of  a  die  set. 

Open-Die  Forging 

Open-die  forging  is  sometimes  referred  to  as  smith  forging  and  is  actually  a  develop- 
ment or  a  modern  version  of  a  very  old  type  of  forging,  blacksmithing,  that  was  prac- 
ticed by  armor  makers  and  crafts  people.  Blacksmithing  required  hand  tools  and  was 
carried  out  by  striking  the  heated  part  repeatedly  by  a  hammer  on  an  anvil  until  the  de- 
sired shape  was  finally  obtained.  Nowadays,  blacksmith  forging  is  used  only  when  low 
production  of  light  forgings  is  required,  which  is  mainly  in  repair  shops.  Complicated 
shapes  having  close  tolerances  cannot  be  produced  economically  by  this  process. 

The  modern  version  of  blacksmithing,  open-die  forging,  involves  the  substitution 
of  a  power-actuated  hammer  or  press  for  the  arm,  hand  hammer,  and  anvil  of  the  smith. 
This  process  is  used  for  producing  heavy  forgings  weighing  up  to  more  than  300  tons, 
as  well  as  for  producing  small  batches  of  medium  forgings  with  irregular  shapes  that 
cannot  be  produced  by  modern  closed-die  forging.  The  skill  of  the  operator  plays  an 
important  role  in  achieving  the  desired  shape  of  the  part  by  manipulating  the  heated 
metal  during  the  period  between  successive  working  strokes.  Accordingly,  the  shape 
obtained  is  just  an  approximation  of  the  required  one,  and  subsequent  machining  is  al- 
ways used  in  order  to  produce  the  part  that  accurately  conforms  to  the  blueprint  pro- 
vided by  the  designer. 

Open-die  forging  operations.  A  smith-forging  process  usually  consists  of  a  group  of 
different  operations.  Among  the  operations  employed  in  smith  forging  are  upsetting, 
drawing  out,  fullering,  cutting  off,  and  piercing.  The  force  and  energy  required  differ 
considerably  from  one  operation  to  another,  depending  upon  the  degree  of  "confine- 
ment" of  the  metal  being  worked.  Following  is  a  brief  description  of  some  of  these 
operations: 

1.  Upsetting.  Upsetting  involves  squeezing  the  billet  between  two  flat  surfaces,  thus 
reducing  its  height  due  to  the  increase  in  the  cross-sectional  area.  As  can  be  seen  in 
Figure  5.42a,  the  state  of  stress  is  uniaxial  compression.  In  practice,  however,  the 
billets'  surfaces  in  contact  with  the  die  are  subjected  to  substantial  friction  forces 
that  impede  the  flow  of  the  neighboring  layers  of  metal.  This  finally  results  in  a  het- 
erogeneous deformation  and  in  barreling  of  the  deformed  billet.  To  obtain  uniform 
deformation,  the  billet-die  interfaces  must  be  adequately  lubricated. 

2.  Drawing  out.  In  drawing  out,  the  workpiece  is  successively  forged  along  its  length 
between  two  dies  having  limited  width.  This  results  in  reducing  the  cross-sectional 


178 


5      Metal  Forming 


FIGURE    5.42 

Various  smith-forging 
operations:  (a) 
upsetting;  (b)  drawing 
out;  (c)  piercing  a  short 
billet;  (d)  piercing  a 
long  billet;  (e)  cutting 
off;  (f)  bending 


(b) 


(a) 


(c) 


I — . — 1  L — [ — __l    I   _  _j  _  -  1 


(d) 


r >< 


Thinning 


Upsetting 


(f) 


area  of  the  workpiece  while  increasing  its  length,  as  shown  in  Figure  5.42b.  This 
operation  can  be  performed  by  starting  either  at  the  middle  or  at  the  end  of  the 
workpiece.  A  large  reduction  in  the  cross-sectional  area  can  be  achieved  by  reduc- 
ing the  feed  of  the  workpiece.  The  bite  (i.e.,  the  length  of  feed  before  the  working 
stroke)  ranges  between  40  and  75  percent  of  the  width  of  the  forging  die. 

Piercing  operation.  A  piercing  operation  is  performed  in  order  to  obtain  blind  or 
through  holes  in  the  billet.  A  through  hole  can  be  pierced  directly  in  a  short  billet 
in  a  single  stroke  by  employing  a  punch  and  a  supporting  ring,  as  shown  in  Fig- 
ure 5.42c.  On  the  other  hand,  billets  with  large  height-to-diameter  ratios  are 
pierced  while  located  directly  on  the  die  with  the  help  of  a  piercer  and  possibly 
an  extension  piece  as  well,  as  shown  in  Figure  5.42d.  In  this  latter  case,  the  di- 
ameter of  the  piercer  must  not  exceed  50  percent  of  that  of  the  billet.  For  larger 
holes,  hollow  punches  are  employed.  Also,  holes  can  be  enlarged  by  tapered 
punches. 


5.5  Forging 


179 


4.  Cutting  off.  Cutting  off  involves  cutting  the  workpiece  into  separate  parts  using  a 
forge  cutter  or  a  suitable  chisel.  This  is  usually  done  in  two  stages,  as  can  be  seen 
in  Figure  5.42e. 

5.  Bending.  In  bending,  thinning  of  the  metal  occurs  on  the  convex  side  at  the  point 
of  localized  bending  (where  bending  actually  takes  place).  It  is,  therefore,  recom- 
mended to  upset  the  metal  at  this  location  before  bending  is  performed,  as  shown 
in  Figure  5.42f,  in  order  to  obtain  a  quality  bend. 

Examples  of  open-die  forged  parts.  As  mentioned  before,  a  part  may  require  a  series 
of  operations  so  that  it  can  be  given  the  desired  shape  by  smith  forging.  Following  are 
some  examples  of  smith-forged  industrial  components,  together  with  the  steps  in- 
volved in  the  manufacture  of  each  part: 

1.  Large  motor  shaft.  First,  24-inch-square  (60  cm)  steel  ingots  are  rolled  into 
square  blooms,  each  having  a  12-inch  (30-cm)  side.  The  blooms  are  then  heated 
and  hammered  successively  across  the  corners  until  the  workpiece  is  finally 
rounded  to  a  diameter  of  10  inches  (25  cm).  These  steps  are  illustrated  in  Fig- 
ure 5.43. 

2.  Flange  coupling.  The  sequence  of  operations  is  illustrated  in  Figure  5.44.  There  are 
two  operations  or  stages  involved,  upsetting  and  heading.  In  heading,  the  flow  of 
metal  of  most  of  the  billet  is  restricted  by  using  a  ring-shaped  tool.  This  process  al- 
lows excellent  grain  flow  to  be  obtained,  which  is  particularly  advantageous  in  car- 
rying tangential  loads. 


FIGURE    5.43 

The  production  of  a 
large  motor  shaft  by 
smith  forging 


Rotate  - 


Rotate 


C?  a 


l^»^^ J       L^ — >. 1       L/ — ■ 1 


FIGURE    5.44 

The  production  of 
flange  coupling  by 
smith  forging 


1 

1 

m       m 


J 1 


180 


Metal  Forming 


3.  Rings.  A  billet  is  first  upset  and  is  then  subjected  to  a  piercing  operation.  This  is 
followed  by  an  expanding  operation  using  a  mandrel  to  reduce  the  thickness  of 
the  ring  and  increase  its  diameter  as  required.  Larger  rings  are  usually  expanded 
on  a  saddle.  The  steps  involved  in  the  process  of  ring  forging  are  illustrated  in 
Figure  5.45. 

Equipment  for  smith  forging.  Smaller  billets  are  usually  smith-forged  using  pneu- 
matic-power hammers.  Larger  components  are  worked  in  steam-power  hammers  (or 
large  pneumatic  hammers),  whereas  very  large  and  heavy  parts  are  produced  by  em- 
ploying hydraulic  presses.  Following  is  a  brief  description  of  smith-forging  equipment: 

1.  Steam-power  hammers.  A  steam-power  hammer  consists  mainly  of  the  moving 
parts  (including  the  ram,  the  rod,  and  the  piston);  a  lifting  and  propelling  device, 
which  is  a  double-acting  high-pressure  steam  cylinder;  the  housing  or  frame,  which 
can  be  either  an  arch  or  an  open  type;  and  the  anvil.  Figure  5.46  illustrates  the 
working  principles.  First,  the  piston  and  the  other  moving  parts  are  raised  by  ad- 
mitting steam  into  the  lower  side  of  the  cylinder  (under  the  piston)  through  the 


FIGURE    5.45 

The  production  of  large 
rings  by  smith  forging 


l 


Saddle 


Upsetting 


Piercing 


FIGURE    5.46 

The  working  principles 
of  a  steam-power 
hammer 


Steam 


r^ 


5.5  Forging  181 

sliding  valve.  When  a  blow  is  required,  the  lever  is  actuated;  the  sliding  valve  is  ac- 
cordingly shifted  to  admit  steam  to  the  upper  side  of  the  cylinder  (above  the  piston) 
and  exhaust  the  steam  that  was  in  the  lower  side,  thus  pushing  the  moving  parts 
downward  at  a  high  speed.  In  steam-power  hammers,  the  velocity  of  impact  can  be 
as  high  as  25  feet  per  second  (3  m/s),  whereas  the  mass  of  the  moving  parts  can  be 
up  to  11,000  slugs  (5000  kg).  The  amount  of  energy  delivered  per  blow  is,  there- 
fore, extremely  large  and  can  be  expressed  by  the  equation: 

E  =  '/2  mV2  (5.15) 

where:  E  is  the  energy 

m  is  the  mass  of  the  moving  parts 
V  is  the  impact  velocity 

Nevertheless,  not  all  of  that  energy  is  consumed  in  the  deformation  of  the  work- 
piece.  The  moving  parts  rebound  after  impact,  and  the  anvil  will  try  to  move  in  the 
opposite  direction,  thus  consuming  or  actually  wasting  a  fraction  of  the  blow  en- 
ergy. The  ratio  between  the  energy  absorbed  in  deforming  the  metal  to  that  deliv- 
ered by  the  blow  is  called  the  efficiency  of  a  hammer  and  can  be  given  by  the 
following  equation: 

M  7 

T\=—J—{\-K2)  (5.16) 

M  +  m 

where:  M  is  the  mass  of  the  anvil 

A'  is  a  factor  that  depends  upon  the  elasticity  of  the  billet 

The  harder  and  more  elastic  the  billet  is,  the  higher  that  factor  will  be,  and  the 
lower  the  efficiency  becomes.  In  addition,  the  hammer  efficiency  depends  upon  the 
ratio  MI{M  +  m),  or  actually  the  ratio  between  the  masses  of  the  anvil  and  the  mov- 
ing parts,  which  is  taken  in  practice  between  15  and  20.  On  the  other  hand,  the 
value  of  K  ranges  between  0.05  and  0.25. 

2.  Pneumatic-power  hammers.  There  are  two  kinds  of  pneumatic-power  hammers. 
The  first  kind  includes  small  hammers  in  which  the  air  compressor  is  built  in;  they 
usually  have  open  frames  because  their  capacity  is  limited.  The  second  kind  of 
pneumatic  hammer  is  generally  similar  to  a  steam-power  hammer  in  construction 
and  operation,  the  only  difference  being  that  steam  is  replaced  by  compressed  air  (7 
to  8  times  the  atmospheric  pressure).  As  is  the  case  with  steam,  this  necessitates 
separate  installation  for  providing  compressed  air.  Pneumatic  hammers  do  not  have 
some  of  the  disadvantages  of  steam  hammers,  such  as  dripping  of  water  resulting 
from  condensation  of  leakage  steam  onto  the  hot  billet.  This  may  result  in  cracking 
of  the  part,  especially  when  forging  steel. 

3.  Hydraulic  presses.  Heavy  forgings  are  worked  in  hydraulic  presses.  The  press  in- 
stallation is  composed  of  the  press  itself  and  the  hydraulic  drive.  Presses  capable  of 
providing  a  force  of  75,000  tons  (750  MN)  are  quite  common.  Still,  hydraulic 
presses  that  are  commonly  used  in  the  forging  industry  have  capacities  ranging  be- 
tween 1000  tons  (10  MN)  and  10,000  tons  (100  MN).  These  presses  can  success- 


182  5      Metal  Forming 


fully  handle  forgings  weighing  between  8  and  250  tons.  The  large-capacity  presses 
require  extremely  high  oil  pressure  in  the  hydraulic  cylinders  (200  to  300  times  the 
atmospheric  pressure).  Because  no  pump  can  deliver  an  adequate  oil  discharge  at 
that  pressure  level,  this  process  is  usually  overcome  by  employing  accumulators 
and  intensifies  that  magnify  the  oil  pressure  delivered  by  the  pump  by  a  factor  of 
40  or  even  60. 

Planning  the  production  of  a  smith-forged  part.  Before  actually  smith  forging  a  part, 
all  the  details  of  the  process  must  be  thoroughly  planned.  This  involves  preparation 
of  the  design  details,  calculation  of  the  dimensions  and  the  weight  of  the  stock  and 
of  the  product,  choosing  the  forging  operations  as  well  as  their  sequence,  choosing 
tools  and  devices  that  will  be  used,  and  thinking  about  the  details  of  the  heating  and 
cooling  cycles. 

The  first  step  in  the  design  process  is  to  draw  the  finished  part  and  then  obtain  the 
drawing  of  the  forging  by  adding  a  machining  as  well  as  a  forging  allowance  all 
around.  The  machining  allowance  is  the  increase  in  any  dimension  to  provide  excess 
metal  that  is  removed  by  machining.  This  subsequent  machining  is  required  to  remove 
scales  and  the  chilled,  defected  surface  layers.  The  forging  allowance  is  added  mainly 
to  simplify  the  shape  of  the  as-forged  part.  It  is  always  recommended  to  make  the 
shape  of  a  forging  symmetrical  and  confined  by  plane  and  cylindrical  surfaces.  At  this 
stage,  a  suitable  tolerance  is  assigned  to  each  dimension  to  bring  the  design  process  to 
an  end. 

The  next  step  is  to  choose  the  appropriate  equipment.  Two  factors  affect  the  deci- 
sion: the  size  of  the  forging  and  the  rate  of  deformation  (strain  rate).  Usually,  forgings 
weighing  2  tons  or  more  are  forged  in  hydraulic  presses.  Also,  small  forging  made  of 
high-alloy  steels  and  some  nonferrous  alloys  must  be  forged  on  a  press  because  they 
are  sensitive  to  high  strain  rates  that  arise  when  using  power-hammer  forging.  At  this 
point,  the  manufacturing  engineer  is  in  a  position  to  decide  upon  operations,  tools,  de- 
vices, and  the  like  needed  to  accomplish  the  desired  task. 

Closed-Die  Forging 

Closed-die  forging  involves  shaping  the  hot  forging  stock  in  counterpart  cavities  or  im- 
pressions that  have  been  machined  into  two  mating  halves  of  a  die  set.  Under  impact 
(or  squeezing),  the  hot  metal  plastically  flows  to  fill  the  die  cavity.  Because  the  flow  of 
metal  is  restricted  by  the  shape  of  the  impressions,  the  forged  part  accurately  conforms 
to  the  shape  of  the  cavity,  provided  that  complete  filling  of  the  cavity  is  achieved. 
Among  the  various  advantages  of  closed-die  forging  are  the  greater  consistency  of 
product  attributes  than  in  casting,  the  close  tolerances  and  good  surface  finish  with 
minimum  surplus  material  to  be  removed  by  machining,  and  the  greater  strength  at 
lower  unit  weight  compared  with  castings  or  fabricated  parts.  In  fact,  the  cost  of  parts 
produced  by  machining  (only)  is  usually  two  to  three  times  the  cost  of  closed-die  forg- 
ings. Nevertheless,  the  high  cost  of  forging  dies  (compared  with  patterns,  for  example) 
is  the  main  shortcoming  of  this  process,  especially  if  intricate  shapes  are  to  be  pro- 
duced. Therefore,  the  process  is  recommended  for  mass  or  large-lot  production  of  steel 
and  nonferrous  components  weighing  up  to  about  900  pounds  (350  kg). 


5.5  Forging  183 

Generally,  there  are  two  types  of  closed-die  forging:  conventional  (or  flash)  die 
forging  and  flashless  die  forging.  In  conventional  flash  die  forging,  the  volume  of  the 
slug  has  to  be  slightly  larger  than  that  of  the  die  cavity.  The  surplus  metal  forms  a  flash 
(fin)  around  the  parting  line.  In  flashless  forging,  no  fin  is  formed,  so  the  process  con- 
sequently calls  for  accurate  control  of  the  volume  of  the  slug.  If  the  slug  is  smaller  than 
the  required  final  product,  proper  filling  of  the  die  cavity  is  not  achieved.  On  the  other 
hand,  when  the  size  of  the  slug  is  bigger  than  that  of  the  desired  forging,  excessive 
load  buildup  will  eventually  result  in  the  breaking  of  the  tooling  and/or  equipment.  Ac- 
cordingly, flashless-forging  dies  are  fitted  with  load-limiting  devices  to  keep  the  gen- 
erated load  below  a  certain  safe  value  in  order  to  avoid  breakage  of  the  tooling. 

In  addition  to  shaping  the  metal  in  die  cavities,  the  manufacturing  cycle  for  a  die- 
forged  part  includes  some  other  related  operations,  such  as  cutting  or  cropping  the 
rolled  stock  into  slugs  or  billets,  adequately  heating  the  slugs,  forging  the  slugs,  trim- 
ming the  flash  (in  conventional  forging),  heat  treating  the  forgings,  descaling,  and,  fi- 
nally, inspecting  or  quality  controlling.  The  forging  specifications  differ  from  one 
country  to  another;  however,  in  order  to  ensure  the  product  quality,  one  or  more  of  the 
following  acceptance  tests  must  be  passed: 

1.  Chemical  composition  midway  between  the  surface  and  the  center 

2.  Mechanical  properties 

3.  Corrosion  tests 

4.  Nondestructive  tests  like  magnetic  detection  of  surface  or  subsurface  hair  cracks 

5.  Visual  tests  such  as  macroetch  and  macroexamination  and  sulfur  painting  for  steel 

Closed-die  forging  processes  can  be  carried  out  using  drop  forging  hammers,  me- 
chanical crank  presses,  and  forging  machines.  Factors  such  as  product  shape  and  tol- 
erances, quantities  required,  and  forged  alloys  play  an  important  role  in  determining 
the  best  and  most  economical  equipment  to  be  employed  in  forging  a  desired  product 
as  each  of  the  processes  has  its  own  advantages  and  limitations.  Following  is  a  brief 
description  of  the  different  techniques  used  in  closed-die  forging. 

Drop  forging.  In  drop  forging,  a  type  of  closed-die  forging,  the  force  generated  by  the 
hammer  is  caused  by  gravitational  attraction  resulting  from  the  free  fall  of  the  ram.  The 
ram  may  be  lifted  by  a  single-acting  steam  (or  air)  cylinder  or  by  friction  rollers  that  en- 
gage a  board  tightly  fastened  to  the  ram.  In  this  latter  type,  called  a  board  hammer,  once 
the  ram  reaches  a  predetermined  desired  height,  a  lever  is  actuated,  the  rollers  retract, 
and  the  board  and  ram  fall  freely  to  strike  the  workpiece.  Figure  5.47  illustrates  the 
working  principles.  Whether  a  board  hammer  or  single-acting  steam  hammer  is  used, 
accurate  matching  of  the  two  halves  of  the  die  (i.e.,  the  impressions)  must  be  ensured. 
Therefore,  the  hammers  employed  in  drop  forging  are  usually  of  the  double-housing  (or 
arch)  type  and  are  provided  with  adequate  ram  guidance.  The  desired  alignment  of  the 
two  halves  of  the  die  is  then  achieved  by  wedging  the  upper  half  of  the  die  onto  the  ram 
and  securing  the  lower  half  onto  a  bolster  plate  that  is,  in  turn,  tightly  mounted  on  the 
anvil.  Also,  the  ratio  of  the  weights  of  the  anvil  and  the  moving  parts  can  go  as  high  as 
30  to  1  to  ensure  maximum  efficiency  and  trouble-free  impact. 


184 


5      Metal  Forming 


FIGURE    5.47 

The  working  principles 
of  a  board  hammer 


Wooden 
board 

Friction 
roller 


^ 


/-^Z^\ 


/-^^\ 


Drop-forging  dies  can  have  one,  two,  or  several  impressions,  depending  upon  the 
complexity  of  the  required  product.  Simple  shapes  like  gears,  small  flywheels,  and 
straight  levers  are  usually  forged  in  dies  with  one  or  two  impressions,  whereas  prod- 
ucts with  intricate  shapes  are  successively  worked  in  multiple-impression  dies,  thus 
making  it  possible  to  preshape  a  forging  before  it  is  forged  into  its  final  form.  Opera- 
tions like  edging,  drawing  out,  fullering,  and  bending  are  performed,  each  in  its  as- 
signed impression.  Finally,  the  desired  shape  is  imparted  to  the  metal  in  a  finishing 
impression  that  has  exactly  the  same  shape  as  the  desired  product;  its  dimensions  are 
slightly  larger  because  shrinkage  due  to  cooling  down  must  be  taken  into  account.  As 
can  be  seen  in  Figure  5.48,  a  gutter  for  flash  is  provided  around  the  finishing  impres- 
sions. When  properly  designed,  the  gutter  provides  resistance  to  the  flow  of  metal  into 


FIGURE    5.48 

A  gutter  providing  a 
space  for  excess  metal 


Gutter 


A  forging  in 

the  finishing 

impression 


Upper  die  half 


Lower  die  half 


Flash 


5.5  Forging 


185 


it,  thus  preventing  further  flow  from  the  impression  and  forcing  the  metal  to  fill  all  the 
details,  such  as  corners  (which  are  the  most  difficult  portions  to  fill). 

The  drop-forging  process  may  involve  several  blows  so  that  the  desired  final 
shape  of  the  forged  part  can  be  obtained.  Lubricants  are  applied  to  ensure  easy  flow  of 
the  metal  within  the  cavity  and  to  reduce  friction  and  die  wear.  As  many  as  four  blows 
may  be  needed  while  the  part  is  in  the  finishing  impressions,  and  the  part  should  be 
lifted  slightly  between  successive  blows  to  prevent  overheating  of  the  die.  Finally,  the 
gas  pressure  forces  the  part  out  of  the  die.  The  number  of  blows  delivered  when  the 
part  is  in  the  different  preshaping  impressions  is  1  Vi  to  2  times  the  number  of  blows 
while  the  part  is  in  the  finishing  impression.  This  sequence  of  drop-forging  operations 
is  shown  when  forging  a  connecting  rod.  As  can  be  seen  in  Figure  5.49,  the  heated 
stock  is  first  placed  in  the  fullering  impression  and  then  hammered  once  or  twice  to  ob- 
tain local  spreading  of  the  metal  on  the  expanse  of  its  cross  section.  The  stock  is  then 
transferred  to  the  edging  impression,  where  the  metal  is  redistributed  along  its  length 
in  order  to  properly  fill  the  finishing  die  cavities  (i.e.,  metal  is  "gathered"  at  certain 
predetermined  points  and  reduced  at  some  other  ones).  This  is  usually  achieved 
through  a  series  of  blows,  together  with  turnovers  of  the  metal,  as  required.  The  next 
operation  in  this  sequence  is  bending,  which  may  or  may  not  be  needed,  depending 
upon  the  design  of  the  product.  The  stock  is  then  worked  in  the  semifinishing,  or 
blocking,  impression  before  it  is  finally  forged  into  the  desired  shape  in  the  finishing 
impression.  We  can  see  that  the  blocking  operation  contributes  to  reducing  the  tool 
wear  in  the  finishing  impression  by  giving  the  part  its  general  shape. 

Press  forging.  Press  forging,  which  is  usually  referred  to  as  hot  pressing,  is  carried 
out  using  mechanical  (crank-type)  or  hydraulic  presses.  These  exert  force  at  relatively 
slow  ram  travel,  resulting  in  steadily  applied  pressure  instead  of  impacting  pressure. 


FIGURE    5.49 

A  multiple-impression 
die  and  the  forging 
sequence  for  a 
connecting  rod 


Initial  forging  stock 


Blocking 


Finishing 


186 


Metal  Forming 


FIGURE    5.50 

Flash  and  flashless  hot 
pressing 


Flash 


Forging 


Ejector 

Flash  hot  pressing 


Forging 


Ejector 
Flashless  hot  pressing 


The  nature  of  metal  deformation  during  hot  pressing  is,  therefore,  substantially  differ- 
ent from  that  of  drop  forging.  Under  impact  loading,  the  energy  is  transmitted  into  only 
the  surface  layers  of  the  workpiece,  whereas,  under  squeezing  (steadily  applied  pres- 
sure), deformation  penetrates  deeper  so  that  the  entire  volume  of  the  workpiece  simul- 
taneously undergoes  plastic  deformation.  Although  multiple-impression  dies  are  used, 
it  is  always  the  goal  of  a  good  designer  to  minimize  the  number  of  impressions  in  a 
die.  It  is  also  considered  good  industrial  practice  to  use  shaped  blanks  or  preforms, 
thus  enabling  the  part  to  be  forged  in  only  a  single  stroke. 

Hot  pressing  involves  both  flash  as  well  as  flashless  forging.  In  both  cases,  the 
forged  part  is  pushed  out  of  the  die  cavity  by  means  of  an  ejector,  as  is  illustrated  in 
Figure  5.50.  Examples  of  some  hot-pressed  parts  are  shown  in  Figure  5.51,  which  also 
shows  the  sequence  of  operations,  the  production  rate,  the  estimated  die  life,  and  the 
approximate  production  cost. 

A  characterizing  feature  of  hot  pressing  is  the  accurate  matching  of  the  two  halves 
of  a  die  due  to  the  efficient  guidance  of  the  ram.  Also,  the  number  of  working  strokes 
per  minute  can  be  as  high  as  40  or  even  50.  There  is  also  the  possibility  of  automating 
the  process  through  mechanization  of  blank  feeding  and  of  forging  removal.  It  can, 
therefore,  clearly  be  seen  that  hot  pressing  has  higher  productivity  than  drop  forging 
and  yields  parts  with  greater  accuracy  in  terms  of  tolerances  within  0.010  to  0.020  inch 
(0.2  up  to  0.5  mm),  less  draft,  and  fewer  design  limitations.  Nevertheless,  the  initial 
capital  cost  is  higher  compared  with  drop  forging  because  the  cost  of  a  crank  press  is 
always  higher  than  that  of  an  equivalent  hammer  and  because  the  process  is  economi- 
cal only  when  the  equipment  is  efficiently  utilized.  The  difficulty  of  descaling  the 
blanks  is  another  shortcoming  of  this  process.  However,  this  disadvantage  can  be  elim- 
inated by  using  hydraulic  descaling  (using  a  high-pressure  water  jet)  or  can  be  origi- 
nally avoided  by  using  heating  furnaces  with  inert  atmosphere. 

Die  forging  in  a  horizontal  forging  machine.  Although  originally  developed  for  head- 
ing operations,  the  purpose  of  this  machine  has  been  broadened  to  produce  a  variety  of 
shapes.  For  instance,  all  axisymmetric  parts  such  as  rods  with  flanges  (with  through 


5.5  Forging 


187 


FIGURE    5.51 

Examples  of  hot-pressed  parts 


Break  lever 

Aluminum 

Die  life 

Cost  in  cents  per  piece 
4.9 

(Bicycle) 

alloy 

40,000  pieces 

Slug(H) 

Forming 

Trimming 

J^ 

Sizing      ^^O*' 

Piercing 

Bearing  race 


(Bearing) 


SAE-5     00 


Die  life 
30,000 


Cost  in  cents  per  piece 
11.2 


Slug 


Upsetting 


Piercing 


n^ 


Backward  extrusion 


,+0.5 

h« 1 — *-i 


73.2^ 


Sizing 


1  ~l  28.88+°  5  mm 


Valve 


(Gas  equipment) 


Slug(H) 


Brass 


Upsetting 


Die  life 
40,000 


Forming 


Cost  in  cents  per  piece 
7.7 


60.1 


Trimming  ™^1" 


and  blind  holes)  and/or  side  projections  are  commonly  produced  on  horizontal  forging 
machines.  A  rolled  stock  is  cut  to  length,  heated  in  a  heating  unit,  and  automatically 
fed  to  the  machine.  As  can  be  seen  in  Figure  5.52,  the  hot  part  is  then  held  by  station- 
ary grips  (actually  a  split  die)  and  upset  by  an  upsetting  ram  or  header.  The  process  in- 
volves mainly  upsetting  and  gathering  where  the  blank  is  first  upset;  then  metal  flows 
to  fill  the  die  cavity,  as  opposed  to  drop  forging,  where  it  is  spread  or  flattened.  In  the 
return  stroke,  the  upsetting  ram  retracts,  and  the  part  is  removed  or  transferred  to  the 
next  impression  of  the  horizontal  forging  machine.  It  is  obvious  that  a  part  can  be 
forged  in  one  or  several  cavities,  depending  upon  the  complexity  of  its  shape. 

The  main  advantage  of  this  process  is  the  high  production  rate  (up  to  5000  parts 
per  hour)  due  to  the  fact  that  it  can  be  fully  automated.  Further  advantages  include  the 


188 


Metal  Forming 


FIGURE    5.52 

Die  forging  in  a 
horizontal  forging 
machine 


Grip  die 


Upsetting 
.tool 


Ejector 


Grip  die 


elimination  of  the  flash  and  the  forging  draft  and  the  high  efficiency  of  material  uti- 
lization because  the  process  involves  little  or  no  waste. 

Recent  Developments  in  Forging 

Warm  forging,  high-energy-rate  forging,  and  forming  of  metals  in  their  mushy  state  are 
among  the  important  developments  in  forging  technology.  These  newly  developed 
processes  are  usually  carried  out  to  obtain  intricate  shapes  or  unique  structures  that 
cannot  be  obtained  by  conventional  forging  processes.  Following  is  a  brief  description 
of  each  of  these  processes,  together  with  their  advantages  and  disadvantages. 

Warm  forging.  Warm  forging  involves  forging  of  the  metal  at  a  temperature  some- 
what below  the  recrystallization  temperature.  This  process  combines  some  advantages 
of  both  the  hot  and  the  cold  forming  processes  while  eliminating  their  shortcomings. 
On  one  hand,  increased  plasticity  and  lower  load  requirements  are  caused  by  the  rela- 
tively high  forging  temperature.  On  the  other  hand,  improved  mechanical  properties, 
less  scaling,  and  longer  die  life  are  due  to  the  lower  temperatures  used  as  compared 
with  those  used  with  hot  forging. 

High-energy-rate  forging.  The  conventional  forging  process  takes  some  time,  during 
which  the  hot  metal  cools  down  and  its  resistance  to  deformation  increases.  As  this 
does  not  occur  with  high-energy-rate  forging  (HERF),  where  the  whole  process  is  per- 
formed within  a  few  thousandths  of  a  second,  the  hot  metal  does  not  have  enough  time 
to  cool  down  and  heat  is  not  dissipated  into  the  surroundings.  Therefore,  HERF  is  very 
successful  when  forging  intricate  shapes  with  thin  sections.  A  special  HERF  machine 
must  be  used.  In  fact,  the  Petro-Forge  machine  was  developed  at  the  Mechanical  En- 
gineering Department  of  Birmingham  University  in  England  for  this  reason,  and  a 
bulky  machine  with  the  name  Dynapak  was  developed  in  the  United  States.  In  the  first 
case,  the  machine  consists  mainly  of  an  internal-combustion  (IC)  cylinder  integrated 
into  the  structure  of  a  high-speed  press.  The  IC  cylinder  is  provided  with  a  sudden  re- 
lease valve  that  allows  the  platen  attached  to  the  piston  to  be  fired  instantaneously 
when  the  combustion  pressure  reaches  a  preset  level.  The  four  stages  of  the  working 


5.5  Forging 


189 


FIGURE    5.53 

The  working  cycle  of  the 
Petro-Forge 


cycle  of  the  Petro-Forge  are  shown  in  Figure  5.53.  In  the  case  of  the  Dynapak,  high- 
pressure  nitrogen  in  a  power  cylinder  is  used  to  push  the  platen  downward.  Installa- 
tions to  produce  and  keep  high-pressure  gas  are,  therefore,  required  in  this  case. 

Forging  of  alloys  in  their  mushy  state.  Forging  alloys  in  their  mushy  state  involves 
plastically  forming  alloys  in  the  temperature  range  above  the  solidus  line.  Because 
an  alloy  at  that  temperature  consists  partly  of  a  liquid  phase,  a  remarkable  decrease 
in  the  required  forging  load  is  experienced.  The  process  also  has  some  other  merits, 
such  as  the  high  processing  rate  and  the  high  quality  of  products  compared  with 
castings.  Moreover,  the  friction  at  the  billet-container  interface  has  been  found  to  be 
almost  negligible.  Nevertheless,  the  process  is  still  considered  to  be  in  its  experi- 
mental stage  because  of  the  instability  of  alloys  having  low  solid  fractions.  Recently, 
it  was  reported  that  progress  has  been  made  toward  solving  this  problem  at  the  In- 
stitute of  Industrial  Science,  Tokyo  University,  where  the  instability  was  overcome 


Injection 


Charging  Working  stroke 


Return  stroke 


Oil  sump 


<*>     J 


il  mist 


Injection 

At  the  beginning  of  the  firing  cycle  the  ram/piston  assembly 
(A)  is  held  at  the  top  of  its  stroke  by  low  pressure  air  in  the 
back  pressure  chamber  (B)  closing  the  combustion  chamber 
porting  by  the  seal  (C),  this  being  a  cylindrical  projection  on 
the  top  face  of  the  piston  (A).  The  exhaust  valve  (D)  is  open 
and  pressure  in  the  combustion  chamber  (E)  is  atmospheric. 
Upon  pressing  the  firing  button  the  fuel  injection  phase 
starts;  the  exhaust  valve  (D)  is  closed  and  the  gaseous  fuel 
is  admitted  into  the  combustion  chamber  (E)  via  the  gas 
valve  (F). 


Working  stroke 

As  soon  as  the  force  due  to  the  combustion  pressure  acting 
on  the  small  area  (I)  on  top  of  the  seal  (C)  is  sufficiently 
large  to  overcome  the  opposed  force  due  to  the  low  back 
pressure  in  the  space  (B)  acting  on  the  annular  lower  face 
of  the  piston,  the  piston  (A)  starts  to  move.  As  a  result  the 
porting  between  the  combustion  chamber  (E)  and  the  cylinder 
is  opened  and  the  gases  are  permitted  to  expand  to  act  over 
the  whole  piston  area.  This  results  in  a  large  force  surge  acting 
on  the  piston/ram  assembly  which  is  accelerated  downwards 
to  impinge  on  the  workpiece. 


Charging 

After  closing  the  gas  valve  (F)  the  combustion  chamber  is 
charged  by  admitting  compressed  air  through  the  inlet  valve 
(G).  As  soon  as  charging  is  completed,  the  inlet  valve  (G)  is 
closed  and  the  air/gas  mixture  is  ignited  by  the  spark  plug 
(H).  This  results  in  a  seven  to  eightfold  rise  of  the  pressure 
in  the  combustion  chamber  (E). 


Return  stroke 

During  the  working  stroke  the  back  pressure  in  space  (B)  is 
intensified  and  consequently  acts  as  a  return  spring  as  soon  as 
the  forming  operation  is  completed,  thus  rapidly  separating 
the  dies.  The  return  of  the  ram/piston  assembly  to  its  initial 
position  is  completed  by  the  opening  of  the  exhaust  valve  (D) 
which  permits  gases  to  leave  through  the  duct  (J).  The  cycle 
of  operation  is  normally  completed  in  one  second. 


190  5      Metal  Forming 


by  dispersing  a  very  fine  alumina  powder.  This  also  yielded  improved  mechanical 
properties  of  forgings. 

Forgeability 

For  the  proper  planning  of  a  forging  process,  it  is  important  to  know  the  deformation  be- 
havior of  the  metal  to  be  forged  with  regard  to  the  resistance  to  deformation  and  any  an- 
ticipated adverse  effects,  such  as  cracking.  For  this  reason,  the  term  forgeability  was 
introduced  and  can  be  defined  as  the  tolerance  of  a  metal  for  deformation  without  failure. 
Although  there  is  no  commonly  accepted  standard  test,  quantitative  assessment  of  the 
forgeability  of  a  metal  (or  an  alloy)  can  be  obtained  through  one  of  the  following  tests. 

Upsetting  test.  The  upsetting  test  involves  upsetting  a  series  of  cylindrical  billets 
having  the  same  dimensions  to  different  degrees  of  deformation  (reductions  in  height). 
The  maximum  limit  of  upsettability  without  failure  or  cracking  (usually  peripheral 
cracks)  is  taken  as  a  measure  of  forgeability. 

Notched-bar  upsetting  test.  The  notched-bar  upsetting  test  is  basically  similar  to  the 
first  test,  except  that  longitudinal  notches  or  serrations  are  made  prior  to  upsetting.  It 
is  believed  that  this  test  provides  a  more  reliable  index  of  forgeability. 

Hot-impact  tensile  test.  A  conventional  impact-testing  machine  fitted  with  a  tension- 
test  attachment  is  employed.  A  hot  bar  of  the  metal  to  be  studied  is  tested,  and  the  im- 
pact tensile  strength  is  taken  as  a  measure  of  forgeability.  This  test  is  recommended 
when  studying  the  forgeability  of  alloys  that  are  sensitive  to  high  strain  rates. 

Hot  twist  test.  The  hot  twist  test  involves  twisting  a  round,  hot  bar  and  counting  the 
number  of  twists  until  failure.  The  greater  the  number  of  twists,  the  better  the  forge- 
ability is  considered  to  be.  Using  the  same  bar  material,  this  test  can  be  performed  at 
different  temperatures  in  order  to  obtain  the  forging  temperature  range  in  which  the 
forgeability  of  a  metal  is  maximum. 

Forgeability  of  Some  Alloys 

It  is  obvious  that  the  results  of  any  of  the  preceding  tests  are  affected  by  factors  like 
the  composition  of  an  alloy,  the  presence  of  impurities,  the  grain  size,  and  the  number 
of  phases  present.  These  are  added  to  the  effect  of  temperature,  which  generally  im- 
proves forgeability  up  to  a  certain  limit,  where  other  phases  start  to  appear  or  where 
grain  growth  becomes  excessive.  At  this  point,  any  further  increase  in  temperature  is 
accompanied  by  a  decrease  in  forgeability.  Following  is  a  list  indicating  the  relative 
forgeability  of  some  alloys  in  descending  order  (i.e.,  alloys  with  better  forgeability  are 
mentioned  first): 

1.  Aluminum  alloys 

2.  Magnesium  alloys 

3.  Copper  alloys 

4.  Plain-carbon  steels 


5.5  Forging  191 

5.  Low-alloy  steels 

6.  Martensitic  stainless  steel 

7.  Austenitic  stainless  steel 

8.  Nickel  alloys 

9.  Titanium  alloys 

10.  Iron-base  superalloys 

11.  Cobalt-base  superalloys 

12.  Molybdenum  alloys 

13.  Nickel-base  superalloys 

14.  Tungsten  alloys 

15.  Beryllium 

Lubrication  in  Forging 

In  hot  forging,  the  role  of  lubricants  is  not  just  limited  to  eliminating  friction  and  en- 
suring easy  flow  of  metal.  A  lubricant  actually  prevents  the  hot  metal  from  sticking  to 
the  die  and  meanwhile  prevents  the  surface  layers  of  the  hot  metal  from  being  chilled 
by  the  relatively  cold  die.  Therefore,  water  spray,  sawdust,  or  liners  of  relatively  soft 
metals  are  sometimes  employed  to  prevent  adhesion.  Mineral  oil  alone  or  mixed  with 
graphite  is  also  used,  especially  for  aluminum  and  magnesium  alloys.  Graphite  and/or 
molybdenum  disulfide  are  widely  used  for  plain-carbon  steels,  low-alloy  steels,  and 
copper  alloys,  whereas  melting  glass  is  used  for  difficult-to-forge  alloys  like  alloy 
steel,  nickel  alloys,  and  titanium. 

Defects  in  Forged  Products 

Various  surface  and  body  defects  may  be  observed  in  forgings.  The  kind  of  defect  de- 
pends upon  many  factors,  such  as  the  forging  process,  the  forged  metal,  the  tool  de- 
sign, and  the  temperature  at  which  the  process  is  carried  out.  Cracking,  folds,  and 
improper  sections  are  generally  the  defects  observed  in  forged  products.  Following  is 
a  brief  description  of  each  defect  and  its  causes. 

Cracking.  Cracking  is  due  to  the  initiation  of  tensile  stresses  during  the  forging 
process.  Examples  are  hot  tears,  which  are  peripheral  longitudinal  cracks  experienced 
in  upsetting  processes  at  high  degrees  of  deformation,  and  center  cavities,  which  occur 
in  the  primary  forging  of  low-ductility  steels.  Thermal  cracks  may  also  initiate  in  cases 
when  nonuniform  temperature  distribution  prevails. 

Folds.  In  upsetting  and  heading  processes,  folding  is  a  common  defect  that  is  obvi- 
ously caused  by  buckling.  Folds  may  also  be  observed  at  the  edges  of  parts  produced 
by  smith  forging  if  the  reduction  per  pass  is  too  small. 

Improper  sections.  Improper  sections  include  dead-metal  zones,  piping,  and  turbu- 
lent (i.e.,  irregular  or  violent)  metal  flow.  They  are  basically  related  to  and  caused  by 
poor  tool  design. 


192 


5      Metal  Forming 


Forging  Die  Materials 

During  their  service  life,  forging  dies  are  subjected  to  severe  conditions  such  as  high 
temperatures,  excessive  pressures,  and  abrasion.  A  die  material  must,  therefore,  pos- 
sess adequate  hardness  at  high  temperatures  as  well  as  high  toughness  to  be  able  to 
withstand  the  severe  conditions.  Special  tool  steels  (hot-work  steels  including  one  or 
more  of  the  following  alloying  additives:  chromium,  nickel,  molybdenum,  and  vana- 
dium) are  employed  as  die  materials.  Die  blocks  are  annealed,  machined  to  make  the 
shanks,  hardened,  and  tempered;  then,  impression  cavities  are  sunk  by  toolmakers. 

Fundamentals  of  Closed-Die  Forging  Design 

The  range  of  forged  products  with  respect  to  size,  shape,  and  properties  is  very  wide 
indeed.  For  this  reason,  it  is  both  advisable  and  advantageous  for  the  product  designer 
to  consider  forging  in  the  early  stages  of  planning  the  processes  for  manufacturing  new 
products.  The  forging  design  is  influenced  not  only  by  its  function  and  the  properties 
of  the  material  being  processed  but  also  by  the  kind,  capabilities,  and  shortcomings  of 
the  production  equipment  available  in  the  manufacturing  facilities.  Therefore,  it  is  im- 
possible to  discuss  in  detail  all  considerations  arising  from  the  infinite  combinations  of 
the  various  factors.  Nevertheless,  some  general  guidelines  apply  in  all  cases  and 
should  be  strictly  adhered  to  if  a  sound  forging  is  to  be  obtained.  Following  are  some 
recommended  forging  design  principles. 

Parting  line.  The  plane  of  separation  between  the  upper  and  lower  halves  of  a  closed 
die  set  is  called  the  parting  line.  The  parting  line  can  be  straight,  whether  horizontal  or 
inclined,  or  can  be  irregular,  including  more  than  one  plane.  The  parting  line  must  be 
designated  on  all  forging  drawings  as  it  affects  the  initial  cost  and  wear  of  the  forging 
die,  the  grain  flow  that,  in  turn,  affects  the  mechanical  properties  of  the  forging,  and, 
finally,  the  trimming  procedure  and/or  subsequent  machining  operations  on  the  fin- 
ished part.  Following  are  some  considerations  for  determining  the  shape  and  position 
of  the  parting  line: 

1.  The  parting  line  should  usually  pass  through  the  maximum  periphery  of  the  forging 
mainly  because  it  is  always  easier  to  spread  the  metal  laterally  than  to  force  it  to  fill 
deep,  narrow  die  impressions  (see  Figure  5.54). 


FIGURE    5.54 

Recommended  location 
of  the  parting  line 
(Courtesy  of  the 
Aluminum  Association, 
Inc.,  Washington,  D.C.) 


Preferred 


Less  desirable 


5.5  Forging 


193 


FIGURE    5.55 

Flat-sided  forging  for 
simplifying  the  die 
construction  (Courtesy 
of  the  Aluminum 
Association,  Inc., 
Washington,  D.C.) 


Plane  surface  formed 
/     by  flat  upper  die 


T 


Parting  line 


Contour  of  forging 

formed  by  impression 

in  bottom  die 


FIGURE    5.56 

Using  the  parting  line 
to  promote  the 
alignment  of  the  fibrous 
macrostructure 
(Courtesy  of  the 
Aluminum  Association, 
Inc.,  Washington,  D.C.) 


Grain  structure  is 

ruptured  at  the 

parting  line 


Parting  line 


Undesirable 


These  parting  lines  result  in  metal  flow  patterns 
that  cause  forging  defects 


Most  economical  as  all  of 
the  impression  is  in  one  die 


This  parting  line  should  not  be   — 
above  the  center  of  the  bottom  web 


r 


Parting  at  the  ends 

of  ribs  results  in 

good  grain  structure 


Recommended  -  The  flow  lines  are  smooth  at  stressed  sections 
with  these  parting  lines 


194 


Metal  Forming 


2.  It  is  always  advantageous,  whenever  possible,  to  try  to  simplify  the  die  construction 
if  the  design  is  to  end  up  with  flat-sided  forgings  (see  Figure  5.55).  This  will 
markedly  reduce  the  die  cost  because  machining  is  limited  to  the  lower  die  half. 
Also,  the  possibility  of  mismatch  between  die  halves  is  eliminated. 

3.  If  an  inclined  parting  line  must  exist,  it  is  generally  recommended  to  limit  the  in- 
clination so  that  it  does  not  exceed  75°.  The  reason  is  that  inclined  flashes  may  cre- 
ate problems  in  trimming  and  subsequent  machining. 

4.  A  parting  line  should  be  located  so  that  it  promotes  alignment  of  the  fibrous 
macrostructure  to  fulfill  the  strength  requirement  of  a  forging.  Because  excess 
metal  flows  out  of  the  die  cavity  into  the  gutter  as  the  process  proceeds,  mislocat- 
ing  the  parting  line  will  probably  result  in  irregularities,  as  can  be  seen  in  Figure 
5.56,  which  indicates  the  fibrous  macrostructures  resulting  from  different  locations 
of  the  parting  line. 

5.  When  the  forging  comprises  a  web  enclosed  by  ribs,  as  illustrated  in  Figure  5.57, 
the  parting  line  should  preferably  pass  through  the  centerline  of  the  web.  It  is  also 
desirable,  with  respect  to  the  alignment  of  fibers,  to  have  the  parting  line  either  at 
the  top  or  at  the  bottom  surfaces.  However,  that  desirable  location  usually  creates 
manufacturing  problems  and  is  not  used  unless  the  direction  of  the  fibrous 
macrostructure  is  critical. 

6.  If  an  irregular  parting  line  must  exist,  avoid  side  thrust  of  the  die,  which  will  cause 
the  die  halves  to  shift  away  from  each  other  sideways,  resulting  in  matching  errors. 
Figure  5.58  illustrates  the  problem  of  side  thrust  accompanying  irregular  parting 
lines,  together  with  two  suggested  solutions. 

Draft.  Draft  refers  to  the  taper  given  to  internal  and  external  sides  of  a  closed-die  forg- 
ing and  is  expressed  as  an  angle  from  the  direction  of  the  forging  stroke.  Draft  is  required 
on  the  vast  majority  of  forgings  to  avoid  production  difficulties,  to  aid  in  achieving  desired 
metal  flow,  and  to  allow  easy  removal  of  the  forging  from  the  die  cavity.  It  is  obvious  that 


FIGURE    5.57 

Location  of  the  parting 
line  with  respect  to  a 
web  (Courtesy  of  the 
Aluminum  Association, 
Inc..  Washington,  D.C.) 


Parting  line 


A—1 


Section  AA 


Section  BB 


5.5  Forging 


195 


FIGURE    5.58 

The  problem  of  side 
thrust  accompanying 
irregular  parting  lines 
and  two  suggested 
solutions  (Courtesy  of 
the  Aluminum 
Association,  Inc., 
Washington,  D.C.) 


Die  lock 


Impractical  —  Side  thrust  makes  it  difficult  to  hold  the  dies 
in  match  accurately 


Die  lock 


Not  recommended  —  Dies  with  counterlocks  are  expensive  to 
build  and  troublesome  to  maintain 


Die  lock 


—  Upper  die 

Forging 
plane 

-  Forging 


Bottom  die 


Upper  die 

Forging 
plane 

Counterlock 


Forging 
Bottom  die 


Upper  die 

Forging 
plane 

Forging 
Bottom  die 


Preferred  —  The  best  method  is  to  incline  the  forging  with 
respect  to  the  forging  plane 


the  smaller  the  draft  angle,  the  more  difficult  it  is  to  remove  the  forging  out  of  the  die.  For 
this  reason,  draft  angles  of  less  than  5°  are  not  permitted  if  the  part  is  to  be  produced  by 
drop  forging  (remember  that  there  is  no  ejector  to  push  the  part  out).  Standard  draft  angles 
are  7°,  5°,  3°,  1  °,  and  0°.  A  draft  angle  of  3°  is  usually  used  for  metal  having  good  forge- 
ability,  such  as  aluminum  and  magnesium,  whereas  5°  and  7°  angles  are  used  for  steels, 
titanium,  and  the  like.  It  is  a  recommended  practice  to  use  a  constant  draft  all  over  the  pe- 
riphery of  the  forging.  It  is  also  common  to  apply  a  smaller  draft  angle  on  the  outside  pe- 
riphery than  on  the  inside  one.  This  is  justified  in  that  the  outer  surface  will  shrink  away 
from  the  surface  of  the  die  cavity  as  a  result  of  the  part's  cooling  down,  thus  facilitating 
the  removal  of  the  forging.  Following  are  some  useful  examples  and  guidelines: 

1.  When  designing  the  product,  try  to  make  use  of  the  natural  draft  inherent  in  some 
shapes,  such  as  curved  and  conical  surfaces  (see  Figure  5.59). 

2.  In  some  cases,  changing  the  orientation  of  the  die  cavity  may  result  in  natural  draft, 
thus  eliminating  the  need  for  any  draft  on  the  surfaces  (see  Figure  5.60). 


196 


5      Metal  Forming 


FIGURE    5.59 

Examples  of  the  natural 

draft  inherent  in  some 
shapes 

Parting               / 
line                 v 

Parting 
line 


FIGURE    5.60 

Examples  of  changing 
the  orientation  of  the 
impression  to  provide 
natural  draft 


Parting 
line 


Parting 
line 


FIGURE    5.61 

Methods  for  matching 
the  contours  of  two  die 
impressions  having 
different  depths: 
(a)  increasing  the 
dimension  of  the  upper 
surface;  (b)  using  a 
pad;  (c)  employing  a 
matching  draft 
(Courtesy  of  the 
Aluminum  Association, 
Inc.,  Washington,  D.C.) 


Sometimes,  the  cavity  in  one  of  the  die  halves  (for  instance,  the  upper)  is  shallower 
than  that  in  the  other  half.  This  may  create  problems  in  matching  the  contours  of 
the  two  die  halves  at  the  parting  line.  It  is,  therefore,  recommended  that  one  of  the 
three  methods  illustrated  in  Figure  5.61a,  b,  or  c  be  used.  The  first  method  involves 
keeping  the  draft  the  same  as  in  the  lower  cavity  but  increasing  the  dimension  of 
the  upper  surface  of  the  cavity.  This  results  in  an  increase  in  weight,  and  this  solu- 


^L 


5°  (Ref) 


r 


Parting  line 


:p[ 


«-  Dim  (applies  to 
lower  side  only) 

(a) 


Parting  line 


!*-  Dim  (applies  to 
both  side) 
(b) 


5°"~ 

1/ 

Match 
draft 

«t    ; 

Parting  line     \ 

5°— 

Dim  (ar. 

)plies  tc 

note  that  it  applies  to  the 
original  5°  intersection  and 
not  to  the  subsequent  match 
draft  intersection) 


(c) 


5.5  Forging 


197 


tion  is  limited  to  smaller  cavities.  The  second  method  is  based  on  keeping  the  draft 
constant  in  both  halves  by  introducing  a  "pad"  whose  height  varies  between  0.06 
inch  (1.5  mm)  and  0.5  inch  (12.5  mm),  depending  upon  the  size  of  the  forging.  The 
third  method,  which  is  more  common,  is  to  provide  greater  draft  on  the  shallower 
die  cavity;  this  is  usually  referred  to  as  matching  draft. 

Ribs.  A  rib  is  a  thin  part  of  the  forging  that  is  normal  to  (or  slightly  inclined  to)  the 
forging  plane.  It  is  obvious  that  optimized  lighter  weight  of  a  forging  calls  for  reduc- 
ing the  thickness  of  long  ribs.  However,  note  that  the  narrower  and  longer  the  rib  is, 
the  higher  the  forging  pressure  is  and  the  more  difficult  it  is  to  obtain  a  sound  rib.  It  is 
actually  a  common  practice  to  keep  the  height-to-thickness  ratio  of  a  rib  below  6, 
preferably  at  4.  The  choice  of  a  value  for  this  ratio  depends  upon  many  factors,  such 
as  the  kind  of  metal  being  processed  and  the  forging  geometry  (i.e.,  the  location  of  the 
rib,  the  location  of  the  parting  line,  and  the  fillet  radii).  Figure  5.62  indicates  the  de- 
sirable rib  design  as  well  as  limitations  imposed  on  possible  alternatives. 

Webs.  A  web  is  a  thin  part  of  the  forging  that  is  passing  through  or  parallel  to  the 
forging  plane  (see  Figure  5.63).  Although  it  is  always  desirable  to  keep  the  thickness 
of  a  web  at  the  minimum,  there  are  practical  limits  for  this.  The  minimum  thickness 
of  webs  depends  on  the  kind  of  material  being  worked  (actually  on  its  forging  tem- 
perature range),  the  size  of  forging  (expressed  as  the  net  area  of  metal  at  the  parting 
line),  and  on  the  average  width.  Table  5.3  indicates  recommended  web  thickness  val- 
ues applicable  to  precision  and  conventional  aluminum  forgings.  For  blocking  cavi- 
ties, the  values  given  in  Table  5.3  must  be  increased  by  50  percent.  Also,  for  steels 
and  other  metals  having  poorer  forgeability  than  aluminum,  it  is  advisable  to  increase 
the  values  for  web  thickness.  Thin  webs  may  cause  unfilled  sections,  may  warp  in 
heat  treatment,  and  may  require  additional  straightening  operations;  they  even  cool 
faster  than  the  rest  of  the  forging  after  the  forging  process,  resulting  in  shrinkage, 
possible  tears,  and  distortion. 


FIGURE    5.62 

Parting 

Recommended  rib 

Defect 

line 

^ 

design  (Courtesy  of  the 

location 

v$\ 

Aluminum  Association, 

Parting 

\Il_ 

Inc.,  Washington,  D.C.) 

W//////A 

W//////A 

line 

Avoid 
and 

thin-ledged  ribs 
small  fillet  radii 

Recommended 

Possible  defect 
if  a>  b 


198 


Metal  Forming 


FIGURE    5.63 

The  shape  of  a  web  in 
forging  (Courtesy  of  the 
Aluminum  Association, 
Inc.,  Washington.  D.C.) 


Corner  radii.  There  are  two  main  factors  that  must  be  taken  into  consideration  when 
selecting  a  small  value  for  a  corner  radius.  First,  a  small  corner  radius  requires  a  sharp 
fillet  in  the  die  steel,  which  acts  as  a  stress  raiser;  second,  the  smaller  the  corner  radius, 
the  higher  the  forging  pressure  required  to  fill  the  die  cavity.  In  addition,  some  other 
factors  affect  the  choice  of  the  corner  radius,  such  as  the  distance  from  the  corner  to 
the  parting  line  and  the  forgeability  of  the  metal  being  worked.  The  larger  the  distance 
from  the  parting  line,  the  larger  the  corner  radius  should  be.  Also,  whereas  a  corner  ra- 
dius of  0.0625  inch  (1.5  mm)  is  generally  considered  adequate  for  aluminum  forging, 
a  corner  radius  of  at  least  0.125  inch  (3  mm)  is  used  for  titanium  forgings  of  similar 
shape  and  size.  In  addition,  the  product  designer  should  try  to  keep  the  corner  radii  as 
consistent  as  possible  and  avoid  blending  different  values  for  a  given  shape  in  order  to 
reduce  the  die  cost  (because  there  will  be  no  need  for  many  tool  changes  during  die 
sinking).  Corner  radii  at  the  end  of  high,  thin  ribs  are  critical.  A  rule  of  thumb  states 


5.5  Forging 


199 


TABLE    5.3 

Recommended  size  of 
minimum  web 
thickness 


Up  to  Average  Width 
in.  (m) 

Up  to  Cross-Sectional  Area 
in.2  (m2) 

Web  Thickness 
in.  (mm) 

3  (0.075) 

10  (0.00625) 

0.09  (2.25) 

4  (0.1) 

30  (0.01875) 

0.12  (3) 

6  (0.15) 

60  (0.0375) 

0.16  (4) 

8  (0.2) 

100  (0.0625) 

0.19  (4.75) 

11  (0.275) 

200  (0.125) 

0.25  (6.25) 

14  (0.35) 

350  (0.21875) 

0.31  (7.75) 

18  (0.45) 

550  (0.34375) 

0.37  (9.25) 

22  (0.55) 

850  (0.53125) 

0.44  (11) 

26  (0.65) 

1200  (0.75) 

0.50  (12.5) 

34  (0.85) 

2000  (1.25) 

0.62  (15.5) 

41  (1.025) 

3000  (1.875) 

0.75  (18.75) 

47  (1.1175) 

4000  (2.50) 

1.25  (31.25) 

52  (1.3) 

5000  (3.125) 

2.00  (50) 

that  it  is  always  desirable  to  have  the  rib  thickness  equal  to  twice  the  value  of  the  cor- 
ner radius.  A  thicker  rib  may  have  a  flat  edge  with  two  corner  radii,  each  equal  to  the 
recommended  value.  Figure  5.64  illustrates  these  recommendations  regarding  corner 
radii  for  ribs. 

Fillet  radii.  It  is  of  supreme  importance  that  the  product  designer  allow  generous  radii 
for  the  fillets  because  abrupt  diversion  of  the  direction  of  metal  flow  can  result  in  nu- 
merous defects  in  the  product.  Figure  5.65  indicates  the  step-by-step  initiation  of  forg- 
ing defects  and  shows  that  small  fillets  result  in  separation  of  the  metal  from  the  die 
and  initiation  of  voids.  Although  these  can  be  filled  at  a  later  stage,  laps  and  cold  shuts 
will  replace  these  voids.  When  the  shape  of  the  part  to  be  forged  is  intricate  (i.e.,  in- 
volving thin  ribs  and  long,  thin  webs),  the  metal  may  preferentially  flow  into  the  gut- 
ter rather  than  into  the  die  cavity.  This  results  in  a  shear  in  the  fibrous  macrostructure 
and  is  referred  to  as  flow-through.  This  latter  defect  can  be  avoided  by  using  larger- 
than-normal  fillets. 


FIGURE    5.64 

Recommendations 
regarding  corner  radii 
for  ribs  (Courtesy  of  the 
Aluminum  Association, 
Inc.,  Washington,  D.C.) 


Straight 


-ur* 


Recommended 
values  of  radii 


t  =  2/? 
Recommended 


Acceptable 


200 


5      Metal  Forming 


FIGURE    5.65 

Defects  caused  by 
employing  smaller  fillet 
radii  (Courtesy  of  the 
Aluminum  Association 
Inc.,  Washington,  D.C.) 


Large  fillets  Forging  stock 

MS 


Die  motion 


Metal  does  not 

hug 
i  sharp  corner 


Metal  reaches 
bottom  of 

cavity  before 
ing  section 


These  cold  shuts 
flawed  in  the 
forging 


Punchout  holes.  Punchout  holes  are  through  holes  in  a  thin  web  that  are  produced 
during,  but  not  after,  the  forging  process.  Punchouts  reduce  the  net  projected  area  of 
the  forging,  thus  reducing  the  forging  load  required.  If  properly  located  and  designed, 
they  can  be  of  great  assistance  in  producing  forgings  with  thin  webs.  In  addition  to  the 
manufacturing  advantages  of  punchouts,  they  serve  functional  design  purposes,  such 
as  reducing  the  mass  of  a  forging  and/or  providing  clearance.  Following  are  some 
guidelines  regarding  the  design  of  punchouts: 

1.  Try  to  locate  a  punchout  around  the  central  area  of  a  thin  web,  where  the  frictional 
force  that  impedes  the  metal  flow  is  maximum. 

2.  Whenever  possible,  use  a  gutter  around  the  interior  periphery  of  a  punchout.  This 
provides  a  successful  means  for  the  surplus  metal  to  escape. 

3.  A  single  large  punchout  is  generally  more  advantageous  than  many  smaller  ones 
that  have  the  same  area.  Accordingly,  try  to  reduce  the  number  of  punchouts  unless 
more  are  dictated  by  functional  requirements. 


5.6  Cold  Forming  Processes  201 

4.  Although  punchouts  generally  aid  in  eliminating  the  problems  associated  with  the 
heat  treatment  of  forgings,  it  may  prove  beneficial  to  take  the  limitations  imposed 
by  heat  treatment  processes  into  account  when  designing  the  contour  of  a  punchout 
(i.e.,  try  to  avoid  irregular  contours  with  sharp  corners). 

Pockets  and  recesses.  Pockets  and  recesses  are  used  to  save  material,  promote  the 
desirable  alignment  of  the  fibrous  macrostructure,  and  improve  the  mechanical  proper- 
ties by  reducing  the  thickness,  thus  achieving  a  higher  degree  of  deformation.  Follow- 
ing are  some  guidelines: 

1.  Recesses  should  never  be  perpendicular  to  the  direction  of  metal  flow. 

2.  Recesses  are  formed  by  punches  or  plugs  in  the  dies.  Therefore,  the  recess  depth  is 
restricted  to  the  value  of  its  diameter  (or  to  the  value  of  minimum  transverse  di- 
mension for  noncircular  recesses). 

3.  Simple  contours  for  the  recesses,  together  with  generous  fillets,  should  be  tried. 


OLD  FORMING  PROCESSES 


Cold  forming  processes  are  employed  mainly  to  obtain  improved  mechanical  proper- 
ties, better  surface  finish,  and  closer  tolerances.  Several  cold  forming  techniques  have 
found  wide  industrial  application.  Among  these  are  sizing,  swaging,  coining,  and  cold 
heading.  Following  is  a  brief  description  of  each  of  them. 

Sizing 

Sizing  (see  Figure  5.66a)  is  a  process  in  which  the  metal  is  squeezed  in  the  forming  di- 
rection but  flows  unrestricted  in  all  transverse  directions.  This  process  is  used  primar- 
ily for  straightening  forged  parts,  improving  the  surface  quality,  and  obtaining  accurate 
dimensions.  A  sizing  operation  can  ensure  accuracy  of  dimensions  within  0.004  up  to 
0.010  inch  (0.1  up  to  0.25  mm).  Meanwhile,  the  pressure  generated  on  the  tools  can  go 
up  to  180,000  pounds  per  square  inch  (1300  MN/m2). 

Swaging 

Swaging  (see  Figure  5.66b)  involves  imparting  the  required  shape  and  accurate  di- 
mensions to  the  entire  forging  (or  most  of  it).  Usually,  swaging  is  carried  out  in  a  die 
where  a  flash  is  formed  and  subsequently  removed  by  abrasive  wheels  or  a  trimming 
operation.  Note  that  the  flow  of  metal  in  the  swaging  process  is  more  restricted  than  in 
sizing.  Accordingly,  higher  forming  pressures  are  experienced  and  can  go  up  to 
250,000  pounds  per  square  inch  (1800  MN/m2). 

Coining 

Coining  (see  Figure  5.66c)  is  a  process  in  which  the  part  subjected  to  coining  is  com- 
pletely confined  within  the  die  cavity  (by  the  die  and  the  punch).  The  volume  of  the 
original  forging  must  be  very  close  to  that  of  the  finished  part.  Any  tangible  increase 
in  that  volume  may  result  in  excessive  pressures  and  the  breakage  of  tools.  Still,  com- 


202 


Metal  Forming 


FIGURE    5.66 

Cold  forming 
processes:  (a)  sizing; 
(b)  swaging;  (c)  coining 


(b) 


mon  pressures  (even  when  no  problems  are  encountered)  are  in  the  order  of  320,000 
pounds  per  square  inch  (2200  MN/nV).  For  this  reason,  coining  processes  (also  sizing 
and  swaging)  are  carried  out  on  special  presses  called  knuckle  presses.  The  main 
mechanism  of  a  knuckle  press  is  shown  in  Figure  5.67.  It  is  characterized  by  the  abil- 
ity to  deliver  a  large  force  with  a  small  stroke  of  the  ram. 

Cold  Heading 

Cold  heading  is  used  to  manufacture  bolts,  rivets,  nuts,  nails,  and  similar  parts  with 
heads  and  collars.  A  group  of  typical  products  are  illustrated  in  Figure  5.68.  The  main 
production  equipment  involves  a  multistage  automatic  cold  header  that  operates  on  the 


5.6  Cold  Forming  Processes 


203 


FIGURE    5.67 

The  working  principles 
of  a  knuckle  press  for 
cold  forming  processes 


.\\\\\K\\\\N 


same  principle  as  a  horizontal  forging  machine.  Full  automation  and  high  productivity 
are  among  the  advantages  of  this  process.  Products  having  accurate  dimensions  can  be 
produced  at  a  rate  of  30  to  300  pieces  per  minute.  Starting  from  coiled  wires  or  rods 
made  of  plain-carbon  steel  and  nonferrous  metals  with  diameters  ranging  from  0.025 
to  1.6  inches  (0.6  to  40  mm),  blanks  are  processed  at  different  stations.  Feeding,  trans- 
fer, and  ejection  of  the  products  are  also  automated.  Figure  5.69  illustrates  the  differ- 
ent stages  involved  in  a  simple  cold  heading  operation. 

Lubrication  in  Cold  Forming 

Lubricants  employed  in  cold  forming  are  similar  to  those  used  in  heavy  wire-drawing 
processes.  Phosphating  followed  by  soap  dipping  is  successful  with  steels,  whereas 
only  soap  is  considered  adequate  for  nonferrous  metals. 


FIGURE    5.68 

Some  products 
manufactured  using  an 
automatic  cold  header 


204 


Metal  Forming 


FIGURE    5.69 

Different  stages  of  a 
simple  cold  heading 
operation 


£ 


3-    € 


3- 


Review  Questions 


,v 


1.  Why  have  metal  forming  processes  gained 
widespread  industrial  application  since  World 
War  II? 

2.  What  are  the  two  main  groups  of  metal  form- 
ing processes? 

3.  List  the  different  factors  affecting  the  defor- 
mation process.  Tell  how  each  influences  de- 
formation. 

4.  Why  are  cold  forming  processes  always  ac- 
companied by  work-hardening,  whereas  hot 
forming  processes  are  not? 

5.  What  is  meant  by  the  fibrous  macro  structure? 

6.  Are  the  mechanical  properties  of  a  rolled  sheet 
isotropic?  Why? 

7.  What  is  meant  by  the  state  of  stress?  List  the 
three  general  types. 

8.  List  some  advantages  of  hot  forming.  What  are 
some  disadvantages? 

9.  List  some  advantages  of  cold  forming.  What 
are  some  disadvantages? 

10.  What  may  happen  when  a  large  section  of 
steel  is  heated  at  a  rapid  rate?  Why? 

11.  What  should  be  avoided  when  heating  large 
steel  sections  prior  to  hot  forming? 

12.  Where  does  friction  occur  in  metal  forming? 

13.  What  are  the  harmful  effects  of  friction  on  the 
forming  process? 

14.  Is  friction  always  harmful  in  all  metal  forming 
processes? 


15.  Can  lead  be  used  as  a  lubricant  when  forming 
copper?  Why? 

16.  When  forming  lead  at  room  temperature,  do 
you  consider  it  cold  forming?  Why? 

17.  Why  are  lubricants  used  in  metal  forming 
processes?  List  some  useful  effects. 

18.  List  some  lubricants  used  in  cold  forming 
processes. 

19.  List  some  lubricants  used  in  hot  forming 
processes. 

20.  Which  do  you  recommend  for  further  process- 
ing by  machining,  a  cold-worked  part  or  a  hot- 
worked  part? 

21.  Is  hot  rolling  the  most  widely  used  metal 
forming  process?  Why? 

22.  List  some  of  the  useful  effects  of  hot  rolling. 

23.  Define  rolling. 

24.  What  is  the  angle  of  contact? 

25.  For  heavier  sections,  would  you  recommend 
larger  angles  of  contact  in  rolling?  Why? 

26.  What  is  the  state  of  stress  in  rolling? 

27.  List  the  different  types  of  rolling  mills. 

28.  What  are  the  different  parts  of  a  roll?  What  is 
the  function  of  each? 

Explain  why  Sendzimir  mills  are  used. 

What  are  universal  mills  used  for? 

31.  List  three  groups  included  in  the  range  of 
rolled  products. 


29 
30 


Chapter  5  Review  Questions 


205 


32.  Explain,  using  sketches,  how  seamless  tubes 
are  manufactured. 

33.  What  is  alligatoring?  What  causes  it? 

34.  Define  wire  drawing. 

35.  Which  mechanical  property  should  the  metal 
possess  if  it  is  to  be  used  in  a  drawing 
process?  Why? 

36.  What  is  the  state  of  stress  in  drawing? 

37.  List  some  advantages  of  the  drawing  process. 

38.  How  is  a  metal  prepared  for  a  drawing  process? 

39.  What  are  the  different  zones  in  a  drawing  die? 

40.  Mention  the  range  of  the  apex  angles  (of  con- 
ical shapes)  used  in  drawing  dies. 

41.  What  material  do  you  recommend  to  be  used 
in  making  drawing  dies? 

42.  Describe  a  draw  bench. 

43.  What  kinds  of  lubricants  are  used  in  drawing 
processes? 

44.  What  is  the  drawing  ratio? 

45.  Give  an  expression  indicating  the  reduction 
achieved  in  a  wire-drawing  process. 

46.  Why  do  internal  bursts  occur  in  wire-drawing 
processes? 

47.  What  are  arrowhead  fractures  and  why  do  they 
occur? 

48.  What  is  the  state  of  stress  in  tube  drawing? 

49.  Using  sketches,  illustrate  the  different  tech- 
niques used  in  tube  drawing. 

50.  Define  extrusion. 

51.  Why  can  extrusion  be  used  with  metals  having 
relatively  poor  plasticity? 

52.  List  some  advantages  of  the  extrusion  process. 

53.  What  are  the  shortcomings  and  limitations  of 
the  extrusion  process? 

54.  Using  sketches,  differentiate  between  the  di- 
rect and  indirect  extrusion  techniques. 

55.  Although  indirect  extrusion  almost  eliminates 
friction,  it  is  not  commonly  used  in  industry. 
Why? 


56.  List  the  advantages  of  hydrostatic  extrusion. 

57.  Compare  extrusion  with  rolling  with  respect  to 
efficiency  of  material  utilization. 

58.  When  is  conventional  direct  extrusion  recom- 
mended as  a  production  process? 

59.  Describe  impact  extrusion. 

60.  Why  is  the  leading  end  of  an  extruded  section 
always  sheared  off? 

61.  What  are  dead-metal  zones? 

62.  If  hardness  measurements  are  taken  across  the 
section  (say,  circular)  of  an  extruded  part, 
what  locations  will  have  higher  hardness  val- 
ues? Can  you  plot  hardness  versus  distance 
from  the  center? 

63.  What  lubricants  can  be  used  in  cold  extrusion? 

64.  What  material  do  you  recommend  as  a  lubri- 
cant when  hot  extruding  stainless  steel? 

65.  What  defect  may  occur  when  extruding  mag- 
nesium at  low  extrusion  ratios? 

66.  What  is  piping  and  why  does  it  occur? 

67.  In  extrusion  dies,  what  is  meant  by  the  circle 
size? 

68.  List  some  considerations  that  must  be  taken 
into  account  when  designing  a  section  for  ex- 
trusion. 

69.  Why  should  a  designer  try  to  avoid  sharp  cor- 
ners at  the  root  of  a  die  tongue?  Explain  using 
neat  sketches. 

70.  As  a  product  designer,  you  are  given  a  very  in- 
tricate section  for  production  by  extrusion.  Is 
there  any  way  around  this  problem  without 
being  forced  to  use  a  die  with  a  very  intricate 
construction?  How? 

71.  List  some  considerations  for  the  design  of  im- 
pact extrusions. 

72.  How  can  you  avoid  shear  failure  at  the  bottom 
of  the  wall  of  an  impact  extrusion? 

73.  Does  forging  involve  just  imparting  a  certain 
shape  to  a  billet? 


206 


Metal  Forming 


75. 

76. 
77. 

78. 
79. 


74.  Is  it  just  a  matter  of  economy  to  produce  a 
crankshaft  by  forging  rather  than  by  machin- 
ing from  a  solid  stock?  Why? 
Can  a  metal  such  as  aluminum  be  forged  at 
any  temperature?  Why? 
List  the  main  types  of  forging  processes. 
Which  process  is  suited  for  the  production  of 
small  batches  of  large  parts? 

Give  examples  of  parts  produced  by  each  type 

of  forging  process.  Support  your  answer  with 

evidence. 

What  is  the  modern  version  of  blacksmi thing? 

What  are  the  different  operations  involved  in 

that  process? 

80.  When  do  you  recommend  using  a  power-actu- 
ated hammer  as  a  forging  machine?  Mention 
the  type  of  forging  process. 

81.  For  which  type  of  forging  is  a  drop  hammer 
employed? 

82.  For  which  type  of  forging  is  a  crank  press 
employed? 

Using  sketches,  illustrate  the  different  stages 
in  manufacturing  a  ring  by  forging. 

List  the  advantages  that  forging  has  over  cast- 
ing when  producing  large  numbers  of  small 
parts  having  relatively  complex  shapes. 

85.  In  the  comparison  of  Question  84,  what  are 
the  shortcomings  of  forging?  Why  don't  they 
affect  your  decision  in  that  particular  case? 

86.  List  some  of  the  specified  acceptance  tests  to 
be  performed  on  forgings. 

87.  What  is  a  board  hammer  used  for? 

88.  Is  it  true  that  a  closed  type  of  forging  die  can 
have  only  one  impression?  Explain  why. 


83. 


84 


89.  What  does  hot  pressing  mean? 

90.  What  is  the  advantage  of  HERF? 

91.  What  are  the  advantages  of  warm  forging? 

92.  What  is  meant  by  a  mushy  state? 

93.  Define  forgeability.  How  can  it  be  quantita- 
tively assessed? 

94.  What  is  the  most  forgeable  metal? 

95.  What  is  the  main  role  of  lubricants  in  hot  forg- 
ing? 

96.  As  a  product  designer,  how  can  you  manipu- 
late the  alignment  of  the  fibrous  macrostruc- 
ture? 

97.  List  some  guidelines  regarding  the  location  of 
the  parting  line  between  the  upper  and  lower 
halves  of  a  die  set. 

98.  What  is  meant  by  the  term  draft  in  forging? 

99.  A  die  was  designed  to  forge  an  aluminum  part. 
Can  the  same  design  be  used  to  forge  a  similar 
part  made  of  titanium?  Why? 

100.  Explain  the  meaning  of  matching  draft,  using 
sketches. 

101.  Differentiate  between  a  web  and  a  rib  in  a 
forging. 

102.  What  is  the  difference  between  a  corner  radius 
and  a  fillet  radius?  Use  sketches. 

103.  What  are  punchout  holes  in  a  forging? 

104.  List  some  advantages  of  including  punchout 
holes  in  a  forging  design. 

105.  Why  are  recesses  sometimes  included  in  a 
forging  design? 

106.  List  the  different  cold  forming  processes  and 
use  sketches  to  illustrate  how  they  differ. 


Chapter  5  Design  Example 


207 


PiohLems 


o, 


1.  In  hot  rolling,  determine  the  load  on  each  roll  of 
a  two-high  rolling  mill,  given  the  following: 


Diameter  of  the 

roll: 

Stock  width: 

Initial  thickness: 

Final  thickness: 

Flow  stress  of 

rolled  material: 


20  inches  (500  mm) 
48  inches  (1020  mm) 
0.08  inch  (2  mm) 
0.04  inch  (1  mm) 

14,200  lb/in.2  (100  MN/m2) 


In  hot  rolling  low-carbon-steel  plate  48  inches 
( 1 200  mm)  in  width,  given  the  roll  diameter  as  20 
inches  (500  mm),  initial  thickness  as  1.5  inches 
(37.5  mm),  final  thickness  as  0.4  inch  (10  mm), 
and  the  flow  stress  of  steel  as  28,400  lb/in.2  (200 
MN/m2),  calculate  the  number  of  rolling  passes  if 
the  maximum  load  on  the  roll  in  each  pass  is  not 
to  exceed  225,000  pounds  force  (1.0  MN). 


I 

3.  Write  a  computer  program  to  solve  Problem  2, 
assuming  that  all  the  data  are  variables  to  be 
given  for  each  design. 

4.  Calculate  the  maximum  achievable  reduction  in 
a  single  drawing  of  a  lead  wire. 

5.  Estimate  the  largest  possible  extrusion  ratio  of 
2.0-inch  (50-mm)  aluminum  bar  having  mean 
flow  stress  of  21,900  lb/in.2  (150  MN/m2)  if  the 
press  available  has  a  capacity  of  only  45,000 
pounds  force  (200  kN). 

6.  Plot  a  curve  indicating  the  efficiency  of  a  drop 
hammer  versus  the  ratio  between  the  weights  of 
the  anvil  and  the  moving  parts  if  the  value  of  K 
that  represents  the  elasticity  of  the  billet  is  taken 
as  0. 1 .  What  ratio  do  you  suggest?  Why  should  it 
not  be  justified  to  take  large  ratios? 


Design  Example 


PROBLEM 

Design  a  simple  wrench  that  measures  1/2  inch  (12.5  mm)  across  bolt-head  flats  and 
is  used  for  loosening  nuts  and  bolts.  The  torque  required  to  loosen  (or  tighten)  a  bolt 
(or  a  nut)  is  1  lb  ft  (6.8  Nm).  The  production  volume  is  25,000  pieces  per  year.  Forg- 
ing is  recommended  as  a  manufacturing  process. 

Solution 

Because  the  wrench  is  going  to  be  short,  it  cannot  be  held  by  the  full  hand  but  prob- 
ably by  only  three  fingers.  The  force  that  can  be  exerted  is  to  be  taken,  therefore,  as 
4  pounds.  The  arm  of  the  lever  is  equal  to  (1  x  12)/4,  or  3  inches  (75mm).  Add  on 
allowance  for  the  holding  fingers.  The  shape  of  the  wrench  will  be  as  shown  in  Fig- 
ure 5.70. 

Now,  let  us  select  the  materials.  A  suitable  material  would  be  AISI  1045  CD  steel 
to  facilitate  machining  (sawing)  of  the  stock  material.  Closed-die  forging  of  the  billets 


208 


5      Metal  Forming 


FIGURE    5.70 

A  wrench  manufactured 
by  forging 


Section  AA 


1875  inch 


0.6  inch 


R=  0.95  inch 


0.375  inch 


;3e 


■Parting  line 


N* 


0.25  inch 


is  recommended,  as  well  as  employing  drop-forging  hammers.  To  facilitate  withdrawal 
of  the  part,  the  cross  section  of  the  handle  should  be  elliptical  (see  Figure  5.70).  The 
parting  line  should  coincide  with  the  major  axis  of  the  ellipse. 
Let  us  check  the  stress  due  to  bending: 


/  =  -n  a3b  =  -  (7t)(0.375)3(0.1875)  =  7.7  x  10" 
4  4 

where:  a  is  half  the  major  axis 
b  is  half  the  minor  axis 


3  in.4 


stress  = 


My 


Ma 
1 


5  x  12  x  0.375 


=  2922  lb/in. 


/  /  7.7  x  10"3 

It  is  less  than  the  allowable  stress  for  1045  CD  steel,  which  is 

^°°°  =  30,000  1M„.' 


In  order  to  check  the  bearing  stress,  let  us  assume  a  shift  of  0.25  inch  between  the 
forces  acting  on  the  faces  of  the  nut  to  form  a  couple  (this  assumption  can  be  verified 
if  we  draw  the  nut  and  the  wrench  to  scale): 


each  force  = 


60 
0.25 


=  240  pounds 


Further  assume  that  the  bearing  area  is  0.375  by  0.25  inch.  The  bearing  stress  is, 
therefore, 


240 


=  2560  lb/in/ 


0.375  x  0.25 
It  is  less  than  the  allowable  stress  of  the  1045  CD  steel. 


Chapter  5  Design  Projects  209 

The  forged  wrench  finally  has  to  be  trimmed  and  then  machined  on  the  surfaces 
indicated  in  Figure  5.70.  An  allowance  of  1/64  inch  should  be  provided  between  the 
wrench  open-head  and  the  nut.  Now,  our  design  is  complete  and  ready  to  be  released 
to  the  workshop. 


ssign  Projects 


BcL 


i 

1.  A  clock  frame  3  by  5  inches  (75  by  125  mm)  is  manufactured  by  machining  an 
aluminum-alloy  stock.  Make  a  design  and  a  preliminary  feasibility  study  so  that  it 
can  be  produced  by  extrusion.  Assume  the  production  volume  is  20,000  pieces  per 
year. 

2.  A  motor  frame  that  has  a  6-inch  (150-mm)  internal  diameter  and  that  is  10  inches 
(250  mm)  long  is  currently  produced  by  casting.  That  process  yields  a  high  per- 
centage of  rejects,  and  the  production  cost  is  relatively  high.  Knowing  that  the  pro- 
duction volume  is  20,000  pieces  per  year,  redesign  the  part  so  that  it  will  be  lighter 
and  can  be  easily  produced  by  an  appropriate  metal  forming  operation  that  has  a 
high  efficiency  of  material  utilization. 

3.  A  pulley  transmits  a  torque  of  600  lb  ft  (816  Nm)  to  a  shaft  that  is  IV4  inches  (31 
mm)  in  diameter.  It  is  to  be  driven  by  a  flat  belt  that  is  2  inches  (50  mm)  in  width. 
Provide  a  detailed  design  for  the  pulley  if  the  production  volume  is  10,000  pieces 
per  year  and  the  pulley  is  manufactured  by  forging. 

4.  A  connecting  lever  is  to  be  manufactured  by  forging.  The  estimated  production  vol- 
ume is  50,000  pieces  per  year.  The  lever  has  two  short  bosses,  each  at  one  of  its 
ends,  and  each  has  a  vertical  hole  3/4  inch  (19  mm)  in  diameter.  The  horizontal  dis- 
tance between  the  centers  of  the  two  holes  is  12  inches  (300  mm),  and  the  vertical 
difference  in  levels  is  3  inches  (75  mm).  The  lever  during  its  functioning  is  sub- 
jected to  a  bending  moment  of  200  lb  ft  (272  Nm).  Make  a  detailed  design  for  this 
lever. 

5.  If  the  lever  in  Problem  4  is  to  be  used  in  a  space  vehicle,  would  you  use  the  same 
material?  What  are  the  necessary  design  changes?  Make  a  design  appropriate  for 
this  new  situation. 

6.  Design  a  gear  blank  that  transmits  a  torque  of  200  lb  ft  (272  Nm)  to  a  shaft  that  is 
3/4  inch  (19  mm)  in  diameter.  The  pitch  diameter  of  the  gear  is  8  inches  (200  mm), 
and  40  teeth  are  to  be  cut  in  that  blank  by  machining.  Assume  the  production  vol- 
ume is  10,000  pieces  per  year. 

7.  A  straight-toothed  spur-gear  wheel  transmits  a  torque  of  1200  lb  ft  (1632  Nm)  to 
a  steel  shaft  (AISI  1045  CD  steel)  that  is  2  inches  (50  mm)  in  diameter.  The  pitch 


210  5      Metal  Forming 


diameter  of  the  gear  is  16  inches  (400  mm),  its  width  is  4  inches  (100  mm),  and 
the  base  diameter  is  15  inches  (375  mm).  Make  a  complete  design  for  this  gear's 
blank  (i.e.,  before  teeth  are  cut)  when  it  is  to  be  manufactured  by  forging.  Assume 
the  production  volume  is  10,000  pieces  per  year. 

A  shaft  has  a  minimum  diameter  of  1  inch  (25  mm)  at  both  its  ends,  where  it  is 
to  be  mounted  in  two  ball  bearings.  The  total  length  of  the  shaft  is  12  inches 
(300  mm).  The  shaft  is  to  have  a  gear  at  its  middle,  with  40  teeth  and  a  pitch- 
circle  diameter  of  1.9  inches  (47.5  mm).  The  width  of  the  gear  is  2  inches  (50  mm). 
Make  a  design  for  this  assembly  if  the  production  volume  is  50,000  per  year. 


Chapter  6 


eet  Metal 
orking 


INTRODUCTION 

The  processes  of  sheet  metal  working  have  recently  gained  widespread  indus- 
trial application.  Their  main  advantages  are  their  high  productivity  and  the  close 
tolerances  and  excellent  surface  finish  of  the  products  (which  usually  require 
no  further  machining).  The  range  of  products  manufactured  by  these  processes 
is  vast,  but,  in  general,  all  of  these  products  have  thin  walls  (relative  to  their 
surface  area)  and  relatively  intricate  shapes.  Sheets  made  from  a  variety  of 
metals  (e.g.,  low-carbon  steel,  high-ductility  alloy  steel,  copper  and  some  of  its 
alloys,  and  aluminum  and  some  of  its  alloys)  can  be  successfully  worked  into 
useful  products.  Therefore,  these  processes  are  continually  becoming  more  at- 
tractive to  the  automotive,  aerospace,  electrical,  and  consumer  goods  indus- 
tries. Products  that  had  in  the  past  always  been  manufactured  by  processes 
like  casting  and  forging  have  been  redesigned  so  that  they  can  be  produced  by 
sheet  metal  working.  Components  like  pulleys,  connecting  rods  for  sewing  ma- 
chines, and  even  large  gears  are  now  within  the  range  of  sheet  metal  products. 

Sheet  metals  are  usually  worked  while  in  their  cold  state.  However,  when 
processing  thick  sheets,  which  are  at  least  0.25  inch  (6  mm)  and  are  referred 
to  as  plates,  thermal  cutting  is  employed  to  obtain  the  required  blank  shape, 
and  the  blank  is  then  hot-worked  in  a  hydraulic  or  friction  screw  press.  Thus, 
fabrication  of  boilers,  tanks,  ship  hulls,  and  the  like  would  certainly  require  hot 
working  of  thick  plates. 

By  far,  the  most  commonly  used  operations  in  sheet  metal  working  are 
those  performed  in  a  press.  For  this  reason,  they  are  usually  referred  to  as 


211 


212 


6      Sheet  Metal  Working 


press  working,  or  simply  stamping,  operations.  Other  techniques  involve  high- 
energy-rate  forming  (HERF),  like  using  explosives  or  impulsive  discharges  of 
electrical  energy  to  form  the  blank,  and  spinning  of  the  sheet  metal  on  a  form 
mandrel.  This  chapter  will  describe  each  of  the  various  operations  employed  in 
sheet  metal  working. 


6.1  PRESS  WORKING  OPERATIONS 


All  press  working  operations  of  sheet  metals  can  be  divided  into  two  main  groups:  cut- 
ting operations  and  shape-forming  operations.  Cutting  operations  involve  separating  a 
part  of  the  blank,  whereas  forming  operations  involve  nondestructive  plastic  deforma- 
tion, which  causes  relative  motion  of  parts  of  the  blank  with  respect  to  each  other.  Cut- 
ting operations  include  shearing,  cutoff,  parting,  blanking,  punching,  and  notching. 
Shape-forming  operations  include  various  bending  operations,  deep  drawing,  emboss- 
ing, and  stretch-forming. 

Cutting  Operations 

The  mechanics  of  separating  the  metal  are  the  same  in  all  sheet  metal  cutting  operations. 
Therefore,  the  operations  are  identified  according  to  the  shape  of  the  curve  along  which 
cutting  takes  place.  When  the  sheet  metal  is  cut  along  a  straight  line,  the  operation  is 
called  shearing  and  is  usually  performed  using  inclined  blades  or  guillotine  shears  in 
order  to  reduce  the  force  required  (see  Figure  6.1).  Cutting  takes  place  gradually,  not  all 
at  once,  over  the  width  of  the  sheet  metal  because  the  upper  blade  is  inclined.  The  angle 
of  inclination  of  the  upper  blade  usually  falls  between  4°  and  8°  and  must  not  exceed  15° 
so  that  the  sheet  metal  is  not  pushed  out  by  the  horizontal  component  of  the  reaction. 

When  cutting  takes  place  along  an  open  curve  (or  on  an  open  corrugated  line),  the 
operation  is  referred  to  as  cutoff,  provided  that  the  blanks  match  each  other  or  can  be 
fully  nested,  as  shown  in  Figure  6.2.  The  cutoff  operation  results  in  almost  no  waste  of 
stock  and  is,  therefore,  considered  to  be  very  efficient  with  respect  to  material  utiliza- 
tion. This  operation  is  usually  performed  in  a  die  that  is  mounted  on  a  crank  press.  If  the 
blanks  do  not  match  each  other,  it  is  necessary  for  cutting  to  take  place  along  two  open 
curves  (or  lines),  as  shown  in  Figure  6.3.  In  this  case,  the  operation  is  called  parting.  It 


FIGURE    6.1 

Shearing  operation  with 
inclined  blades 


Upper 

>ZL      blade 


6.1  Press  Working  Operations 


213 


FIGURE    6.2 

Examples  of  cutoff 
operations 


Strip 


Strip 


lyi* 


Cutting  takes 

place  along  these 

two  lines,  each 

stroke 


Blank 


Final 
blank  shape 


FIGURE    6.3 

An  example  of  a  parting 
operation 


Cutting  takes  place 
simultaneously 
along  these  lines 


Blank 

is  clear  from  the  figure  that  a  parting  operation  results  in  some  waste  of  stock  and  is, 
therefore,  less  efficient  than  shearing  and  cutoff  operations. 

In  blanking  operations,  cutting  occurs  along  a  closed  contour  and  results  in  a 
relatively  high  percentage  of  waste  in  stock  metal,  a  fact  that  makes  blanking  oper- 
ations less  efficient  than  other  cutting  operations.  Nevertheless,  this  process  is  used 
for  mass  production  of  blanks  that  cannot  be  manufactured  by  any  of  the  preceding 
operations.  An  efficient  layout  of  blanks  on  the  strip  of  sheet  metal  can  result  in  an 
appreciable  saving  of  material.  An  example  of  a  good  layout  is  shown  in  Figure 
6.4a,  where  circular  blanks  are  staggered.  The  in-line  arrangement  shown  in  Figure 
6.4b  is  less  efficient  in  terms  of  material  utilization.  Because  a  blanking  operation 
is  performed  in  a  die,  there  is  a  limit  to  the  minimum  distance  between  two  adja- 
cent blanks.  It  is  always  advantageous  to  keep  this  minimum  distance  larger  than  70 
percent  of  the  thickness  of  the  sheet  metal.  In  blanking,  the  part  separated  from  the 
sheet  metal  is  the  product,  and  it  is  usually  further  processed.  But  if  the  remaining 


FIGURE    6.4 

Two  methods  for  laying 
out  circular  blanks  for 
blanking  operations: 

(a)  staggered  layout; 

(b)  in-line  arrangement 


Blank 


°3% 


Strip 


Narrower 
strip 


ooo 
ooo 

QOO 


Blank 


(a) 


(b) 


214 


6      Sheet  Metal  Working 


FIGURE    6.5 

Different  patterns  of 
holes  produced  by 
perforating  operations 


o 
o  o 
o  o 
o   o 

o 


o  o  o  o  o 

o  o  o  o  o 

o  o  o  o  o 

o  o  o  o  o 

o  o  o  o  o 


o   o   o   o   o 


FIGURE    6.6 

Progressive  working 
operations 


Seminotching 
(2)     \    (3) 


Punching 
pilot  holes 


imw  m 


o 


Notching 


Final 
product 


Cut  off  along  this  line 
to  separate  the  product 


part  of  the  sheet  is  required  as  a  product,  the  operation  is  then  termed  punching. 
Sometimes,  it  is  required  to  simultaneously  punch  a  pattern  of  small  holes  as  an  or- 
nament, for  light  distribution,  or  for  ventilation;  the  operation  is  then  referred  to  as 
perforating.  Figure  6.5  illustrates  some  patterns  of  perforated  holes. 

A  notching  operation  is  actually  a  special  case  of  punching,  where  the  removed 
part  is  adjacent  to  the  edge  of  the  strip.  It  is  clear  that  any  required  shape  can  be  ob- 
tained by  carrying  out  several  notching  operations.  For  this  reason,  notching  is  usually 
employed  in  progressive  dies.  A  similar  operation,  called  seminotching,  in  which  the 
separated  part  is  not  attached  to  the  side  of  the  strip,  is  also  used  in  progressive  work- 
ing of  sheet  metals.  In  Figure  6.6,  we  can  see  both  of  these  operations  and  how  they 
can  be  employed  progressively  to  produce  a  blank  with  an  intricate  shape. 

Mechanics  of  sheet  metal  cutting.  Let  us  now  look  further  at  the  process  of  cutting 
sheet  metal.  For  simplicity,  consider  the  simple  case  where  a  circular  punch,  together 
with  a  matching  die,  are  employed  to  punch  a  hole.  Figure  6.7  shows  the  punch,  die, 
and  sheet  metal  during  a  punching  operation.  When  a  load  is  applied  through  the 
punch,  the  upper  surface  of  the  metal  is  elastically  bent  over  the  edge  of  the  punch, 
while  the  lower  surface  is  bent  over  the  edge  of  the  die.  With  further  increase  in  the 
punch  load,  the  elastic  curvature  becomes  permanent  or  plastic  and  is  referred  to  as  the 
rollover.  Next,  the  punch  sinks  into  the  upper  surface  of  the  sheet,  while  the  lower  sur- 
face sinks  into  the  die  hole.  This  stage  involves  mainly  plastic  flow  of  metal  by  shear- 
ing as  there  are  two  forces  equal  in  magnitude  and  opposite  in  direction,  subjecting  the 
cylindrical  surface  within  the  metal  to  intense  shear  stress.  The  result  will  be  a  cylin- 
drical smooth  surface  in  contact  with  the  cylindrical  surface  of  the  punch  as  it  sinks 
into  the  sheet  metal.  Also,  a  similar  surface  forms  the  border  of  the  part  of  the  metal 
sinking  into  the  die  hole.  Each  of  these  smooth  surfaces  is  called  a  burnish.  The  extent 
of  a  burnish  depends  upon  the  metal  of  the  sheet  as  well  as  on  the  design  features  of 


6.1  Press  Working  Operations 


215 


FIGURE    6.7 

Stages  of  a  blanking 
operation 


Punch 


Crack 


Final  hole 


Final  blank 


Burr 

Fracture  surface 

Burnish 

Rollover 


FIGURE    6.8 

Blanking  operations 

where  the  punch-die 

clearance  is: 

(a)  excessive;  (b)  too 

tight 


the  die.  The  burnish  ranges  approximately  between  40  and  60  percent  of  the  stock 
thickness,  the  higher  values  being  for  soft  ductile  materials  like  lead  and  aluminum.  At 
this  stage,  two  cracks  initiate  simultaneously  in  the  sheet  metal,  one  at  the  edge  of  the 
punch  and  the  other  at  the  edge  of  the  die.  These  two  cracks  propagate  and  finally  meet 
each  other  to  allow  separation  of  the  blank  from  the  sheet  metal.  This  zone  has  a  rough 
surface  and  is  called  the  fracture  surface  (break  area).  Finally,  when  the  newly  formed 
blank  is  about  to  be  completely  separated  from  the  stock,  a  burr  is  formed  all  around 
its  upper  edge.  Thus,  the  profile  of  the  edge  of  a  blank  involves  four  zones:  a  rollover, 
a  burnish,  a  fracture  surface,  and  a  burr.  In  fact,  the  profile  of  the  edge  of  the  gener- 
ated hole  consists  of  the  same  four  zones,  but  in  reverse  order. 

We  are  now  in  a  position  to  discuss  the  effects  of  some  process  parameters,  such 
as  the  punch-die  clearance.  Figure  6.8a  illustrates  the  case  where  the  punch-die  clear- 
ance is  excessive  and  is  almost  equal  to  the  thickness  of  the  sheet.  Initially,  the  metal 
is  bent  onto  the  round  edges  of  the  punch  and  the  die,  and  it  then  forms  a  short  circu- 
lar wall  connecting  the  flat  bottom  and  the  bulk  of  the  sheet.  With  further  increase  in 
the  applied  load,  the  wall  elongates  under  the  tensile  stress,  and  tearing  eventually  oc- 
curs. As  can  be  seen  in  Figure  6.8a,  the  blank  resulting  in  this  case  has  a  bent,  torn 
edge  all  around  and,  therefore,  has  no  value.  On  the  other  hand,  if  the  punch-die  clear- 
ance is  too  tight,  as  shown  in  Figure  6.8b,  the  two  cracks  that  initiate  toward  the  end 
of  the  operation  do  not  meet,  and  another  shearing  must  take  place  so  that  the  blank 
can  be  separated.  This  operation  is  referred  to  as  the  secondary  shear.  As  can  be  seen, 
the  obtained  blank  has  an  extremely  rough  side.  In  addition,  the  elastically  recovering 


Location  where 

secondary  shear 

occurs 


ESf-FS3 


Edge  of 
blank 


(b) 


(a) 


216 


6      Sheet  Metal  Working 


FIGURE    6.9 

Elastic  recovery  of  the 
metal  around  the  hole 
gripping  the  punch 


FIGURE    6.10 

Elastic  recovery  of  the 
blank  necessitating  die 
relief 


sheet  stock  tends  to  grip  the  punch,  as  shown  in  Figure  6.9,  thus  increasing  the  force 
required  to  withdraw  the  punch  from  the  hole,  which  is  usually  called  the  stripping 
force.  This  results  in  excessive  punch  wear  and  shorter  tool  life.  On  the  other  hand,  the 
blank  undergoes  elastic  recovery,  and  it  is,  therefore,  necessary  to  provide  relief  by  en- 
larging the  lower  part  of  the  die  hole,  as  shown  in  Figure  6.10. 

Between  these  two  extremes  for  the  punch-die  clearance,  there  exists  an  optimum 
value  that  reduces  or  minimizes  the  stripping  force  and  the  tool  wear  and  also  gives  a 
blank  with  a  larger  burnish  and  smaller  fracture  surface.  This  recommended  value  for 
the  punch-die  clearance  is  usually  taken  as  about  10  to  15  percent  of  the  thickness  of 
the  sheet  metal,  depending  upon  the  kind  of  metal  being  punched. 

Forces  required.  Based  on  the  preceding  discussion,  the  force  required  for  cutting 
sheet  metal  is  equal  to  the  area  subjected  to  shear  stress  (the  product  of  the  perimeter 
of  the  blank  multiplied  by  the  thickness  of  the  sheet  metal)  multiplied  by  the  ultimate 
shear  strength  of  the  metal  being  cut.  The  blanking  force  can  be  expressed  by  the  fol- 
lowing equation: 

F=KxQxtx  xultimate  (6.1) 

where:  Q  is  the  perimeter 
/  is  the  thickness 
^ultimate is  trie  ultimate  shear  strength 

Note  that  K  is  an  experimentally  determined  factor  to  account  for  the  deviation  of  the 
stress  state  from  that  of  pure  shear  and  is  taken  as  about  1.3.  The  ultimate  shear  stress 
can  either  be  obtained  from  handbooks  or  be  taken  as  approximately  0.8  of  the  ulti- 
mate tensile  strength  of  the  same  metal. 

We  can  now  see  that  one  of  the  tasks  of  a  manufacturing  engineer  is  to  calculate  the 
required  force  for  blanking  (or  punching)  and  to  make  sure  that  it  is  below  the  capacity 
of  the  available  press.  This  is  particularly  important  in  industries  that  involve  blanking 
relatively  thick  plates.  There  is,  however,  a  solution  to  the  problem  when  the  required 
force  is  higher  than  the  capacity  of  the  available  press.  It  is  usually  achieved  by  bevel- 
ing (or  shearing)  the  punch  face  in  punching  operations  and  the  upper  surface  of  the  die 
steel  in  blanking  operations.  Shearing  the  punch  results  in  a  perfect  hole  but  a  distorted 
blank,  whereas  shearing  the  die  yields  a  perfect  blank  but  a  distorted  hole.  Nevertheless, 
in  both  cases,  cutting  takes  place  gradually,  not  all  at  once,  along  the  contour  of  the  hole 
(or  the  blank),  with  the  final  outcome  being  a  reduction  in  the  required  blanking  force. 
The  shear  angle  is  usually  taken  proportional  to  the  thickness  of  the  sheet  metal  and 
ranges  between  2°  and  8°.  Double-sheared  punches  are  quite  common  and  are  employed 


6.1  Press  Working  Operations 


217 


to  avoid  the  possibility  of  horizontal  displacement  of  sheet  metals  during  punching.  Fig- 
ure 6. 1 1  illustrates  the  basic  concept  of  punch  and  die  shearing.  It  also  provides  a  sketch 
of  a  double-sheared  punch. 

Another  important  aspect  of  the  punching  (or  blanking)  operation  is  the  stripping 
force  (i.e.,  the  force  required  to  pull  the  punch  out  of  the  hole).  It  is  usually  taken  as 
10  percent  of  the  cutting  force,  although  it  depends  upon  some  process  parameters, 
such  as  the  elasticity  and  plasticity  of  the  sheet  metals,  the  punch-die  clearance,  and 
the  kind  of  lubricant  used. 

Bar  cropping.  Bar  cropping  is  similar  to  sheet  metal  cutting.  Although  bars,  not 
sheets,  are  cut,  the  mechanics  of  the  process  are  similar  to  those  of  sheet  metal  cutting, 
and  separation  of  the  cropped  part  is  due  to  plastic  flow  caused  by  intense  shear  stress. 
The  process  is  used  for  mass  production  of  billets  for  hot  forging  and  cold  forming 
processes.  Nevertheless,  the  distortion  and  work-hardening  at  the  sheared  cross  section 
limit  the  application  of  bar  cropping  when  the  billets  are  to  be  cold  formed.  Therefore, 
a  modified  version  of  the  cropping  operation  has  to  be  used.  It  involves  completely 
confining  the  cropped  billet  and  applying  an  axial  stress  of  approximately  20  percent 
of  the  tensile  strength  of  the  bar  material.  This  bar-cropping  technique,  which  is  shown 
in  Figure  6.12,  yields  a  very  smooth  cropped  surface  and  distortion-free  billets. 

Fine  blanking.  As  we  saw  previously,  the  profile  of  the  edge  of  a  blank  is  not  smooth 
but  consists  of  four  zones:  the  rollover,  the  burnish,  the  fracture  surface  (break  area), 
and  the  burr.  Sometimes,  however,  the  blank  must  have  a  straight,  smooth  side  for 
some  functional  reasons.  In  this  case,  an  operation  called  fine  blanking  is  employed,  as 


FIGURE    6.11 

Shearing  of  the  punch 

and  the  die: 

(a)  sheared  punch 

resulting  in  distorted 

blanks;  (b)  sheared  die 

resulting  in  distorted 

holes 


FIGURE    6.12 

Bar  cropping  with 
workpiece  totally 
confined 


Movable 


Fixed 
blade 


218 


6      Sheet  Metal  Working 


FIGURE    6.13 

Fine-blanking  operation 


Upper  punch 


Pressure  pad 


Sheet  metal 


Die  steel 


Figure  6.13  shows.  This  operation  necessitates  the  use  of  a  triple-action  press  and  a 
special  die  with  a  very  small  punch-die  clearance.  As  can  be  seen  in  the  figure,  the 
metal  is  squeezed  and  restrained  from  moving  in  the  lateral  directions  in  order  to  con- 
trol the  shear  flow  along  a  straight  vertical  direction.  A  variety  of  shapes  can  be  pro- 
duced by  this  method.  They  can  have  any  irregular  outer  contour  and  a  number  of 
holes  as  well.  The  fine-blanking  operation  has  found  widespread  application  in  preci- 
sion industries. 

Miscellaneous  cutting  operations.  The  primary  operation  that  is  used  for  preparing 
strips  for  blanking  is  needed  because  the  available  sheets  vary  in  width  between  32  and 
80  inches  (800  to  2000  mm),  a  range  that  is  usually  not  suitable  because  of  the  di- 
mensions of  the  die  and  the  press.  Therefore,  coils  having  a  suitable  width  have  to  be 
obtained  first.  The  operation  performed  is  called  slitting,  and  it  employs  two  circular 
cutters  for  each  straight  cut.  Sometimes,  slitting  is  carried  out  in  a  rolling  plant,  and 
coils  are  then  shipped  ready  for  blanking. 

A  secondary  operation  that  is  sometimes  carried  out  on  blanks  (or  holes)  to  elim- 
inate rough  sides  and/or  to  adjust  dimensions  is  the  shaving  operation.  The  excess 
metal  in  this  case  is  removed  in  the  form  of  chips.  As  can  be  seen  in  Figure  6.14,  the 
punch-die  clearance  is  very  small.  For  this  reason,  the  die  must  be  rigid,  and  matching 
of  its  two  halves  must  be  carefully  checked. 

Sometimes,  punching  operations  are  mistakenly  called  piercing.  In  fact,  the  me- 
chanics of  sheet  metal  cutting  in  the  two  operations  are  completely  different.  We  can 
see  in  Figure  6.15  that  piercing  involves  a  tearing  action.  We  can  also  see  the  pointed 


FIGURE    6.14 

The  shaving  operation 


Sheet 
meta 


6.1  Press  Working  Operations 


219 


FIGURE    6.15 

The  piercing  operation 


Punch 


ie  steel 


shape  of  the  punch.  Neither  blanks  nor  metal  waste  result  from  the  piercing  operation. 
Instead,  a  short  sleeve  is  generated  around  the  hole,  which  sometimes  has  functional 
application  in  toy  construction  and  the  like. 

Cutting-die  construction.  The  construction  of  cutting  dies  may  take  various  forms. 
The  simplest  one  is  the  drop-through  die,  which  is  shown  in  Figure  6.16.  In  addition 
to  the  punch  and  die  steels,  the  die  includes  the  upper  and  lower  shoes,  the  guideposts, 
and  some  other  auxiliary  components  for  guiding  and  holding  the  metal  strip.  The 
stripper  plate  touches  the  strip  first  and  holds  it  firmly  during  the  blanking  operation; 
it  then  continues  to  press  it  until  the  punch  is  totally  withdrawn  from  the  hole  made  in 
the  strip.  The  generated  blanks  fall  through  the  die  hole,  which  has  a  relief  for  this  rea- 
son, and  are  collected  in  a  container  located  below  the  bed  of  the  press. 

Consequently,  this  die  construction  is  applicable  only  if  the  bed  of  the  press  has  a 
hole.  On  the  other  hand,  if  the  diameter  of  the  required  blanks  is  too  large,  the  use  of 
a  drop-through  die  may  result  in  a  defect  called  dishing.  As  shown  in  Figure  6.17.  this 
defect  involves  slackening  of  the  middle  of  the  blank  in  such  a  manner  that  it  becomes 
curved  and  not  flat.  The  answer  to  this  problem  lies  in  employing  a  return-type  die. 


FIGURE    6.16 

Die  construction  for 
simple  drop-through 
blanking  die 


Spring 

Stripper 
plate 


Die  steel 


Lower 
die  shoe 


220 


6      Sheet  Metal  Working 


FIGURE    6.17 

A  vertical  section 
through  a  blank  with 
the  dishing  defect 


Figure  6.18  shows  that  in  this  type  of  die  construction,  the  blank  is  supported  through- 
out the  operation  by  a  spring-actuated  block  that  finally  pushes  the  blank  upward 
above  the  surface  of  the  strip,  where  it  is  automatically  collected.  A  more  complicated 
die  construction,  like  that  shown  in  Figure  6.19,  can  be  used  to  perform  two  operations 
simultaneously.  This  is  usually  referred  to  as  a  compound  die.  As  can  be  seen  in  Fig- 
ure 6.19,  the  hollow  blanking  punch  is  also  a  hole-punching  die.  This  allows  blanking 
and  punching  to  be  carried  out  simultaneously.  The  product,  which  is  a  washer,  and  the 
central  scrap  are  removed  by  return  blocks. 

Bending  Operations 

Bending  is  the  simplest  operation  of  sheet  metal  working.  It  can,  therefore,  be  carried 
out  by  employing  simple  hand  tools.  As  opposed  to  cutting  operations,  there  is  always 
a  clear  displacement  between  the  forces  acting  during  a  bending  operation.  The  gener- 
ated bending  moment  forces  a  part  of  the  sheet  to  be  bent  with  respect  to  the  rest  of  it 
through  local  plastic  deformation.  Therefore,  all  straight  unbent  surfaces  are  not  sub- 
jected to  bending  stresses  and  do  not  undergo  any  deformation.  Figure  6.20  illustrates 
the  most  commonly  used  types  of  bending  dies:  the  V-type,  the  wiping,  and  the  chan- 
nel (U-type)  dies.  We  can  see  that  the  displacement  between  forces  is  maximum  in  the 


FIGURE    6.18 

A  return-type  die 


Punch 


Stripper 
plate 


FIGURE    6.19 

A  compound  die  for 
producing  a  washer 


Punch  steel 


Stripper 
plate 


Required 
washer 


Die  steel 


6.1  Press  Working  Operations 


221 


FIGURE    6.20 

The  three  common 
types  of  bending  dies: 
(a)  V-type  die;  (b)  wiping 
die;  (c)  channel  (U-type) 
die 


mzzzzzszEEzmtz 


(a) 


(b) 


case  of  the  V-type  die,  and,  therefore,  lower  forces  are  required  to  bend  sheet  metal 
when  using  this  kind  of  die. 

Mechanics  of  bending.  The  bending  of  sheet  metal  resembles  the  case  of  a  beam  with 
a  very  high  width-to-height  ratio.  When  the  load  is  applied,  the  bend  zone  undergoes 
elastic  deformation;  then  plastic  deformation  occurs  with  a  further  increase  in  the  ap- 
plied load.  During  the  elastic  deformation  phase,  the  external  fibers  in  the  bend  zone 
are  subjected  to  tension,  whereas  the  internal  fibers  are  subjected  to  compression.  The 
distribution  of  stresses  is  shown  in  Figure  6.21a.  Note  that  there  is  a  neutral  plane  that 
is  free  of  stresses  at  the  middle  of  the  thickness  of  the  sheet.  The  length  of  the  neutral 
axis  remains  constant  and  does  not  undergo  either  elongation  or  contraction.  Next, 
when  the  plastic  phase  starts,  the  neutral  plane  approaches  the  inner  surface  of  the 
bend,  as  can  be  seen  in  Figure  6.21b.  The  location  of  the  neutral  plane  is  dependent 
upon  many  factors,  such  as  the  thickness  of  the  sheet  metal,  the  radius,  and  the  degree 
of  bend.  Nevertheless,  the  distance  between  the  neutral  plane  and  the  inner  surface  of 
the  bend  is  taken  as  equal  to  40  percent  of  the  thickness  of  the  sheet  metal  as  a  first  ap- 
proximation for  blank-development  calculations. 

Let  us  now  consider  a  very  important  phenomenon — namely,  springback,  which 
is  an  elastic  recovery  of  the  sheet  metal  after  the  removal  of  the  bending  load.  As  Fig- 
ure 6.22  indicates,  for  bending  by  an  angle  of  90°,  the  springback  amounts  to  a  few  de- 
grees. Consequently,  the  obtained  angle  of  bend  is  larger  than  the  required  one.  Even 


FIGURE    6.21 

Distribution  of  stress 
across  the  sheet 
thickness:  (a)  in  the 
early  stage  of  bending; 
(b)  toward  the  end  of  a 
bending  operation 


Bending 
moment 


Neutral 
plane 


Y- 


Tension 


Compression 


Sheet 
metal 


(a) 


(b) 


222 


6      Sheet  Metal  Working 


FIGURE    6.22 

The  springback 
phenomenon 


Position  of  the  sheet  metal 
after  partial  elastic  recovery 


Springback 


toward  the  end  of  the  bending  operation,  the  zone  around  the  neutral  plane  is  subjected 
to  elastic  stresses  and,  therefore,  undergoes  elastic  deformation  (see  Figure  6.21b).  As 
a  result,  the  elastic  core  tries  to  return  to  its  initial  flat  position  as  soon  as  the  load  is 
removed.  When  doing  so,  it  is  impeded  by  the  plastically  deformed  zones.  The  final 
outcome  is,  therefore,  an  elastic  recovery  of  just  a  few  degrees.  Consequently,  the  way 
to  eliminate  springback  involves  forcing  this  elastic  core  to  undergo  plastic  deforma- 
tion. This  can  be  achieved  through  either  of  the  techniques  shown  in  Figure  6.23a  and 
b.  In  the  first  case,  the  punch  is  made  so  that  a  projection  squeezes  the  metal  locally; 
in  the  second  case,  high  tensile  stress  is  superimposed  upon  bending.  A  third  solution 
is  overbending,  as  shown  in  Figure  6.23c.  In  this  case,  the  amount  of  overbending 
should  be  equal  to  the  springback  so  that  the  exact  required  angle  is  obtained  after  the 
elastic  recovery. 

Blank  development.  We  have  previously  referred  to  the  fact  that  the  neutral  plane 
does  not  undergo  any  deformation  during  the  bending  operation  and  that  its  length, 
therefore,  remains  unchanged.  Accordingly,  the  length  of  the  blank  before  bending  can 
be  obtained  by  determining  the  length  of  the  neutral  plane  within  the  final  product.  The 
lengths  of  the  straight  sections  remain  unchanged  and  are  added  together.  The  follow- 
ing equation  can  be  applied  to  any  general  bending  product,  such  as  the  one  shown  in 
Figure  6.24: 


L  =  total  length  of  blank  before  bending 

3       4      180     '       180     2      180     3 


(6.2) 


FIGURE    6.23 

Methods  used  to 
eliminate  springback: 

(a)  bottoming; 

(b)  overbending; 

(c)  stretch-forming 


Bending  moment 


The  final 
required 
position 


(0 


6.1  Press  Working  Operations 


223 


FIGURE    6.24 

A  bending  product 
divided  into  straight 
and  circular  sections 
for  blank  development 


where:  R  is  equal  to  r  +  0.4/ 

r  is  the  inner  radius  of  a  bend 

t  is  the  thickness  of  the  sheet  metal 

R  is  the  radius  of  the  neutral  axis 

Classification  of  bending  operations.  Various  operations  can  be  classified  as  bending, 
although  each  one  has  its  own  industrial  name.  They  include,  for  example,  conven- 
tional bending,  flanging,  hemming,  wiring,  and  corrugating.  The  flanging  operation  is 
quite  similar  to  conventional  bending,  except  that  the  ratio  of  the  lengths  of  the  bent 
part  to  that  of  the  sheet  metal  is  small.  Flanging  is  usually  employed  to  avoid  a  sharp 
edge,  thus  eliminating  the  possibility  of  injury.  It  is  also  used  to  add  stiffness  to  the 
edges  of  sheet  metal  and  for  assembly  purposes. 

Among  the  bending  operations,  hemming  used  to  be  a  very  important  one,  before 
the  recent  developments  in  welding  and  can-forming  technologies.  A  hem  is  a  flange 
that  is  bent  by  180°;  it  is  used  now  to  get  rid  of  a  sharp  edge  and  to  add  stiffness  to 
sheet  metal.  A  few  decades  ago,  hems  were  widely  employed  for  seaming  sheet  met- 
als. Figure  6.25  shows  four  different  kinds  of  hems.  A  similar  operation  is  wiring, 
which  is  shown  in  Figure  6.26.  True  wiring  involves  bending  the  edge  of  the  sheet 
metal  around  a  wire.  Sometimes,  the  operation  is  performed  without  a  wire,  and  it  is 
then  referred  to  as  false  wiring. 

Corrugating  is  another  operation  that  involves  bending  sheet  metal.  Different 
shapes,  like  those  shown  in  Figure  6.27,  are  obtained  by  this  operation.  These  shapes 
possess  better  rigidity  and  can  resist  bending  moments  normal  to  the  corrugated  cross 


FIGURE    6.25 

Different  kinds  of  hems 


J 


Flat  hem 


Open  hem 


Teardrop  hem 


Seaming  using 
two  hems 


224 


6      Sheet  Metal  Working 


FIGURE    6.26 

Wiring  operation 


True  wiring 


False  wiring 


FIGURE    6.27 

Different  shapes  of 
corrugated  sheet  metal 


JU%* 


section  mainly  because  of  the  increase  in  the  moment  of  inertia  of  the  section  due  to 
corrugation  and  because  of  the  work-hardened  zones  resulting  from  bending. 

Miscellaneous  bending  operations.  Conventional  bending  operations  are  usually  car- 
ried out  on  a  press  brake.  However,  with  the  developments  in  metal  forming  theories 
and  machine  tool  design  and  construction,  new  techniques  have  evolved  that  are  em- 
ployed in  bending  not  only  sheet  metal  but  also  iron  angles,  structural  beams,  and 
tubes.  Figure  6.28  illustrates  the  working  principles  and  the  stages  involved  in  roll 
bending.  As  can  be  seen  in  the  figure,  the  rolls  form  a  pyramid-type  arrangement.  Two 
rolls  are  used  to  feed  the  material,  whereas  the  third  (roll  B)  gradually  bends  it  (see 
Figure  6.28a  and  b).  The  direction  of  feed  is  then  reversed,  and  roll  A  now  gradually 
bends  the  beam  (see  Figure  6.28c  and  d). 

Another  bending  operation  that  recently  emerged  and  that  is  gaining  industrial  ap- 
plication is  rotary  bending.  Figure  6.29  illustrates  the  working  principles  of  this  oper- 


FIGURE    6.28 

Stages  involved  in  roll 
bending  a  structural 
beam:  (a)  feeding; 

(b)  initial  bending; 

(c)  further  bending; 

(d)  reversing  the 
direction  of  feed 


Roll  A 


Roll  B 


(b) 


(d) 


FIGURE    6.29 

Working  principles  of 
rotary  bending 


Saddle 


Rocker 


Anvil 


6.1  Press  Working  Operations 


225 


ation.  As  can  be  seen,  the  rotary  bender  includes  three  main  components:  the  saddle, 
the  rocker,  and  the  die  anvil.  The  rocker  is  actually  a  cylinder  with  a  V-notch  along  its 
length.  The  rocker  is  completely  secured  inside  the  saddle  (i.e.,  the  saddle  acts  like  a 
housing)  and  can  rotate  but  cannot  fall  out.  The  rotary  bender  can  be  mounted  on  a 
press  brake.  The  rocker  acts  as  both  a  pressure  pad  and  a  bending  punch.  Among  the 
advantages  claimed  for  rotary  bending  are  the  elimination  of  the  pressure  pad  and  its 
springs  (or  nitrogen  cylinders),  lower  required  tonnage,  and  the  possibility  of  over- 
bending  without  the  need  for  any  horizontal  cams.  This  new  method  has  been  patented 
by  the  Accurate  Manufacturing  Association  and  is  nicknamed  by  industrial  personnel 
as  the  "Pac  Man"  bending  operation. 

A  bending  process  that  is  usually  mistakenly  mentioned  among  the  rolling 
processes  is  the  manufacture  of  thin-walled  welded  pipes.  Although  rolls  are  the  form- 
ing tools,  the  operation  is  actually  a  gradual  and  continuous  bending  of  a  strip  that  is 
not  accompanied  by  any  variation  in  the  thickness  of  that  strip.  Figure  6.30  indicates 
the  basic  principles  of  this  process.  Notice  that  the  width  of  the  strip  is  gradually  bent 
to  take  the  form  of  a  circle.  Strip  edges  must  be  descaled  and  mechanically  processed 
before  the  process  is  performed  to  improve  weldability.  Either  butt  or  high-frequency 
induction  welding  is  employed  to  weld  the  edges  together  after  the  required  circular 
cross  section  is  obtained.  This  process  is  more  economical  and  more  productive  than 
seamless  tube  rolling.  Poor  strength  and  corrosion  resistance  of  seams  are  considered 
as  its  main  disadvantages. 

Deep  Drawing  Operation 

Deep  drawing  involves  the  manufacture  of  deep,  cuplike  products  from  thin  sheet 
metal.  As  can  be  seen  in  Figure  6.31,  the  tooling  basically  involves  a  punch  with  a 
round  corner  and  a  die  with  a  large  edge  radius.  It  can  also  be  seen  that  the  punch-die 
clearance  is  slightly  larger  than  the  thickness  of  the  sheet  metal.  When  load  is  applied 
through  the  punch,  the  metal  is  forced  to  flow  radially  and  sink  into  the  die  hole  to 
form  a  cup.  This  is  an  oversimplification  of  a  rather  complex  problem.  For  the  proper 
design  of  deep-drawn  products  as  well  as  the  tooling  required,  we  have  to  gain  a 
deeper  insight  into  the  process  and  understand  its  mechanics. 

Mechanics  of  deep  drawing.  Consider  what  happens  during  the  early  stages  of  ap- 
plying the  load.  As  Figure  6.32a  shows,  the  blank  is  first  bent  onto  the  round  edge  of 
the  die  hole.  With  further  increase  in  the  applied  load,  the  part  of  the  blank  that  was 
bent  is  straightened  in  order  to  sink  into  the  annular  clearance  between  the  punch  and 


FIGURE    6.30 

Roll  bending  as 
employed  in  the 
manufacture  of  seamed 
tubes 


r"-M 


-i- 

m 
i 


r~\ 


KlS 


w 


ntn 


f- 


f- 


rfi 
r 


* 


11441 


rf!t 


1WJ 


226 


6      Sheet  Metal  Working 


FIGURE    6.31 

Basic  concept  of  deep 
drawing 


Blank  holder 
The  drawn  cup 

Die 


the  die,  thus  forming  a  short,  straight,  vertical  wall.  Next,  the  rest  of  the  blank  starts  to 
flow  radially  and  to  sink  into  the  die  hole,  but  because  the  lower  surface  of  the  blank 
is  in  contact  with  the  upper  flat  surface  of  the  die  steel,  frictional  forces  try  to  impede 
that  flow.  These  forces  are  a  result  of  static  friction;  their  magnitude  drops  as  the  blank 
metal  starts  to  move.  Now  consider  what  happens  to  a  sector  of  the  blank,  such  as  that 
shown  in  Figure  6.32b,  when  its  metal  flows  radially.  It  is  clear  that  the  width  of  the 
sector  shrinks  so  that  the  large  peripheral  perimeter  of  the  blank  can  fit  into  the  smaller 
perimeter  of  the  die  hole.  This  is  caused  by  circumferential  compressive  stresses  act- 
ing within  the  plane  of  the  blank.  With  further  increase  in  the  applied  load,  most  of  the 
blank  sinks  into  the  die  hole,  forming  a  long  vertical  wall,  while  the  remaining  part  of 
the  blank  takes  the  form  of  a  small  annular  flange  (see  Figure  6.31).  The  vertical  wall 
is  subjected  to  uniaxial  tension  whose  magnitude  is  increasing  when  going  toward  the 
bottom  of  the  cup. 

We  can  see  from  the  preceding  discussion  that  the  deep  drawing  process  involves 
five  stages:  bending,  straightening,  friction,  compression,  and  tension.  Different  parts 
of  the  blank  being  drawn  are  subjected  to  different  states  of  stress.  As  a  result,  the  de- 
formation is  not  even  throughout  the  blank,  as  is  clear  in  Figure  6.33,  which  shows  an 
exaggerated  longitudinal  section  of  a  drawn  cup.  While  the  flange  gets  thicker  because 
of  the  circumferential  compressive  stress,  the  vertical  wall  gets  thinner,  and  thinning  is 


FIGURE    6.32 

Mechanics  of  deep 

drawing:  (a)  first  stage 

of  deep  drawing  (i.e., 

bending); 

(b)  compression  stage 

in  deep  drawing 


(a) 


(b) 


6.1  Press  Working  Operations 


227 


FIGURE    6.33 

An  exaggerated 
longitudinal  section  of  a 
drawn  cup,  with  the 
states  of  stress  at 
different  locations 


Maximum  thinning 
occurs  here 


Biaxial 
G     compression, 
thickening 


Uniaxial  tension, 
thinning 


maximum  at  the  lowest  part  of  the  wall  adjacent  to  the  bottom  of  the  cup.  Accordingly, 
if  the  cup  is  broken  during  the  drawing  process,  failure  is  expected  to  occur  at  the  lo- 
cation of  maximum  thinning.  An  upper  bound  for  the  maximum  drawing  force  can, 
therefore,  be  given  by  the  following  equation: 

F  =  K  x  (d  +  t)tCT  (6.3) 

where:  F  is  the  maximum  required  drawing  force 

d  is  the  diameter  of  the  punch 

t  is  the  thickness  of  the  blank 

<3T  is  the  ultimate  tensile  strength  of  the  blank  material 

The  blank  holder.  As  previously  mentioned,  the  thin  blank  is  subjected  to  compres- 
sive stresses  within  its  plane.  This  is  similar  to  the  case  of  a  slender  column  subjected 
to  compression,  where  buckling  is  expected  to  occur  if  the  slenderness  ratio  (i.e., 
length/thickness)  is  higher  than  a  certain  value.  Therefore,  by  virtue  of  similarity,  if  the 
ratio  of  the  diameter  of  the  blank  to  its  thickness  exceeds  a  certain  value,  buckling  oc- 
curs. Actually,  if  (D0  -d)lt>  18,  where  D  is  the  blank  diameter,  d  is  the  punch  diam- 
eter, and  t  is  the  thickness,  the  annular  flange  will  buckle  and  crimple.  This  is  a  product 
defect  referred  to  as  wrinkling. 

One  way  to  eliminate  wrinkling  (buckling)  of  the  thin  blank  is  to  support  it  over 
its  entire  area.  This  is  done  by  sandwiching  the  blank  between  the  upper  surface  of  the 
die  steel  and  the  lower  surface  of  an  annular  ring  that  exerts  pressure  upon  the  blank, 
as  shown  in  Figure  6.31.  This  supporting  ring  is  called  the  blank  holder,  and  the  force 
exerted  on  it  can  be  generated  by  die  springs  or  a  compressed  gas  like  nitrogen.  On  the 
other  hand,  higher  frictional  forces  will  initiate  at  both  the  upper  and  lower  surfaces  of 
the  blank  as  a  result  of  the  blank-holding  force.  For  this  reason,  lubricants  like  soap  in 
water,  waxes,  mineral  oil,  and  graphite  are  applied  to  both  surfaces  of  the  blank.  More- 
over, the  upper  surface  of  the  die  steel  as  well  as  the  lower  surface  of  the  blank  holder 
must  be  very  smooth  (ground  and  lapped).  As  a  rule  of  thumb,  the  blank-holding  force 
is  taken  as  1/3  the  force  required  for  drawing. 


228  6      Sheet  Metal  Working 


Variables  affecting  deep  drawing.  Now  that  we  understand  the  mechanics  of  the 
process,  we  can  identify  and  predict  the  effect  of  each  of  the  process  variables.  For  ex- 
ample, we  can  see  that  poor  lubrication  results  in  higher  friction  forces,  and,  accord- 
ingly, a  higher  drawing  force  is  required.  In  fact,  in  most  cases  of  poor  lubrication,  the 
cup  cross  section  does  not  withstand  the  high  tensile  force,  and  failure  of  the  wall  at 
the  bottom  takes  place  during  the  process.  A  small  die  corner  radius  would  increase  the 
bending  and  straightening  forces,  thus  increasing  the  drawing  force,  and  the  final  out- 
come would  be  a  result  similar  to  that  caused  by  poor  lubrication. 

In  addition  to  these  process  variables,  the  geometry  of  the  blank  has  a  marked  ef- 
fect not  only  on  the  process  but  also  on  the  attributes  of  the  final  product.  An  appro- 
priate quantitative  way  of  expressing  the  geometry  is  the  number  indicating  the 
thickness  as  a  percentage  of  the  diameter,  or  (t/D)  x  100.  For  smaller  values  of  this 
percentage  (e.g.,  0.5),  excessive  wrinkling  should  be  expected,  unless  a  high  blank- 
holding  force  is  used.  If  the  percentage  is  higher  than  3,  no  wrinkling  occurs,  and  a 
blank  holder  is  not  necessary. 

Another  important  variable  is  the  drawing  ratio,  wJaich  is  given  by  the  following 
equation: 

R  =  4  <6-4> 

a 

where:  R  is  the  drawing  ratio 

D  is  diameter  of  the  blank 
d  is  the  diameter  of  the  punch 

It  has  been  experimentally  found  that  the  deep  drawing  operation  does  not  yield  a 
sound  cup  when  the  drawing  ratio  is  higher  than  2  (i.e.,  for  successful  drawing,  R  must 
be  less  than  2). 

Another  number  that  is  commonly  used  to  characterize  drawing  operations  is  the 
percentage  reduction.  It  can  be  given  by  the  following  equation: 

r  =  ^^xl00  (6.5) 

D 

where:  r  is  the  percentage  reduction 
D  is  the  diameter  of  the  blank 
d  is  the  diameter  of  the  punch 

It  is  a  common  industrial  practice  to  take  the  value  of  r  as  less  than  50  percent  in  order 
to  have  a  sound  product  without  any  tearing.  When  the  final  product  is  long  and  neces- 
sitates a  value  of  r  higher  than  50  percent,  an  intermediate  cup  must  be  obtained  first,  as 
shown  in  Figure  6.34.  The  intermediate  cup  must  have  dimensions  that  keep  the  per- 
centage reduction  below  50.  It  can  then  be  redrawn,  as  illustrated  in  Figure  6.35,  once  or 
several  times  until  the  final  required  dimensions  are  achieved.  The  maximum  permissi- 
ble percentage  reduction  in  the  redrawing  operations  is  always  far  less  than  50  percent. 
It  is  usually  taken  as  30  percent,  20  percent,  and  13  percent,  in  the  first,  second,  and  third 
redraws,  respectively.  If  several  redrawing  operations  are  required,  the  product  should 


6.1  Press  Working  Operations 


229 


FIGURE    6.34 

The  use  of  an 
intermediate  cup  when 
the  total  required 
reduction  ratio  is  high 


t  I  ' 'I 

T 

r  =   D  ~d    X  100  >  50 


?  9 

/&ZZZZZZZZZZZ2) 


^zzzzzz^ 


FIGURE    6.35 

Redrawing  an 
intermediate  cup 


Force 


then  be  annealed  after  every  two  operations  in  order  to  eliminate  work-hardening  and 
thus  avoid  cracking  and  failure  of  the  product. 

Blank-development  calculations.  For  the  sake  of  simplicity,  it  is  always  assumed  that 
the  thickness  of  the  blank  remains  unchanged  after  the  drawing  operation.  Because  the 
total  volume  of  the  metal  is  constant,  it  can  then  be  concluded  that  the  surface  area  of 
the  final  product  is  equal  to  the  surface  area  of  the  original  blank.  This  rule  forms  the 
basis  for  the  blank-development  calculations.  Consider  the  simple  example  shown  in 
Figure  6.36.  The  surface  area  of  the  cup  is  the  area  of  its  bottom  plus  the  area  of  the 
wall: 

surface  area  of  cup  =  —d   +  ndh 
4 


FIGURE    6.36 

A  simple  example  of 
blank  development 


VZZZ2ZZZZ0 


-3 


Surface  area  of  blank  -D2  =  -d2  +  ndh,  i.e.,  surface  area  of  the  cup 
4  4 


230  6      Sheet  Metal  Working 


This  is  equal  to  the  surface  area  of  the  original  blank;  therefore,  we  can  state  that 

—Dr  =  —d+  ndh 

4  4 


or 


D2  =  d2  +  4dh 


Therefore,  the  original  diameter  of  the  blank,  which  is  unknown,  can  be  given  by  the 
following  equation: 


D  =  Vd2  +  4dh 


(6.6) 


Equation  6.6  gives  an  approximate  result  because  it  assumes  the  cup  has  sharp  corners, 
which  is  not  the  case  in  industrial  practice.  However,  this  equation  can  be  modified  to 
take  round  corners  into  account  by  adding  the  area  of  the  surface  of  revolution  result- 
ing from  the  rotation  of  the  round  corner  around  the  centerline  of  the  cup,  when  equat- 
ing the  area  of  the  product  to  that  of  the  original  blank.  Note  that  the  area  of  any 
surface  of  revolution  can  be  determined  by  employing  Pappus's  first  theorem,  which 
gives  that  area  as  the  product  of  the  path  of  the  center  of  gravity  of  the  curve  around 
the  axis  of  rotation  multiplied  by  the  length  of  that  curve. 

Planning  for  deep  drawing.  The  process  engineer  usually  receives  a  blueprint  of  the 
required  cup  from  the  product  designer.  His  or  her  job  is  to  determine  the  dimensions 
of  the  blank  and  the  number  of  drawing  operations  needed,  together  with  the  dimen- 
sions of  intermediate  cups,  so  that  the  tool  designer  can  start  designing  the  blanking 
and  the  deep  drawing  dies.  That  job  requires  experience  as  well  as  close  contact  be- 
tween the  product  designer  and  the  process  engineer.  The  following  steps  can  be  of 
great  help  to  beginners: 

1.  Allow  for  a  small  flange  around  the  top  of  the  cup  after  the  operation  is  completed. 
This  flange  is  trimmed  at  a  later  stage  and  is  referred  to  as  the  trimming  allowance.  It 
is  appropriate  to  take  an  allowance  equal  to  10  to  15  percent  of  the  diameter  of  the  cup. 

2.  Calculate  the  total  surface  area  of  the  product  and  the  trimming  allowance.  Then, 
equate  it  to  the  area  of  the  original  blank  with  an  unknown  diameter.  Next,  solve 
for  the  diameter  of  the  original  blank. 

3.  Calculate  the  thickness  as  a  percentage  of  the  diameter  or  (t/D)  x  100,  in  order  to 
get  a  rough  idea  of  the  degree  of  wrinkling  to  be  expected  (see  the  preceding  dis- 
cussion on  process  variables). 

4.  Calculate  the  required  percentage  reduction.  If  it  is  less  than  or  equal  to  50,  then  the 
required  cup  can  be  obtained  in  a  single  drawing.  But  if  the  required  r  is  greater 
than  50,  then  a  few  redrawing  operations  are  required;  the  procedure  to  be  followed 
is  given  in  the  next  steps. 

5.  For  the  first  draw,  assume  r  to  be  equal  to  50  and  calculate  the  dimensions  of  the 
intermediate  cup.  Then,  calculate  r  required  for  the  first  redraw.  If  r  <  30,  only  a 
single  redraw  is  required. 


6.1  Press  Working  Operations 


231 


6.  If  r  >  30  for  the  first  redraw,  take  it  as  equal  to  30  and  calculate  the  dimensions  of 
a  second  intermediate  cup.  The  percentage  reduction  for  the  second  redraw  should 
be  less  than  20;  otherwise,  a  third  redraw  is  required,  and  so  on. 

Ironing.  We  can  see  from  the  mechanics  of  the  deep  drawing  operation  that  there  is 
reasonable  variation  in  the  thickness  of  the  drawn  cup.  In  most  cases,  such  thickness 
variation  does  not  have  any  negative  effect  on  the  proper  functioning  of  the  product, 
and,  therefore,  the  drawn  cups  are  used  as  is.  However,  close  control  of  the  dimensions 
of  the  cups  is  sometimes  necessary.  In  this  case,  cups  are  subjected  to  an  ironing  op- 
eration, in  which  the  wall  of  the  cup  is  squeezed  in  the  annular  space  between  a  punch 
and  its  corresponding  die.  As  can  be  seen  in  Figure  6.37,  the  punch-die  clearance  is 
smaller  than  the  thickness  of  the  cup  and  is  equal  to  the  final  required  thickness.  Large 
reductions  in  thickness  should  be  avoided  in  order  to  obtain  a  sound  product.  It  is  good 
industrial  practice  to  take  the  value  of  the  punch-die  clearance  in  the  range  between  30 
and  80  percent  of  the  thickness  of  the  cup.  Also,  the  percentage  reduction  in  thickness, 
which  is  given  next,  should  fall  between  40  and  60  in  a  single  ironing  operation.  This 
is  a  safeguard  against  fracture  of  the  product  during  the  operation.  Following  is  the 
equation  to  be  applied: 


percentage  reduction  in  thickness  = 


tp-tf 


x  100 


(6.7) 


where:  tQ  is  the  original  thickness  of  the  cup 

tf  is  the  final  thickness  of  the  cup  after  ironing 

Drawing  of  stepped,  conical,  and  domed  cups.  Stepped  cups  are  those  with  two  (or 
more)  shell  diameters  (see  Figure  6.38a).  They  are  produced  in  two  (or  more)  stages. 
First,  a  cup  is  drawn  to  have  the  large  diameter,  and,  second,  a  redrawing  operation  is 
performed  on  only  the  lower  portion  of  the  cup.  Tapered  or  conical  cups  (see  Figure 


FIGURE    6.37 

The  ironing  operation 


FIGURE    6.38 

Deep-drawn  cups: 

(a)  stepped;  (b)  conical; 

(c)  domed 


*ZZZZ\ 


&Z2Z& 


=f 


Tvtv; 


(a) 


(b) 


(O 


232 


6      Sheet  Metal  Working 


6.38b)  cannot  be  drawn  directly.  They  first  have  to  be  made  into  stepped  cups,  which 
are  then  smoothed  and  stretched  out  to  give  the  required  tapered  cups.  A  complex  deep 
drawing  operation  is  used  for  producing  domed  cups  (see  Figure  6.38c).  So  that  the 
sheet  metal  stretches  properly  over  the  punch  nose,  higher  blank-holding  forces  are  re- 
quired. Therefore,  the  process  actually  involves  stretch-forming,  and  its  variables 
should  be  adjusted  to  eliminate  either  wrinkling  or  tearing. 

Drawing  of  box-shaped  cups.  When  all  press  working  operations  of  sheet  metal  are 
reviewed,  there  would  be  almost  no  doubt  that  the  box  drawing  process  is  the  most 
complex  and  difficult  to  control.  Nevertheless,  in  an  attempt  to  simplify  the  problem, 
we  can  divide  a  box  into  four  round  corners  and  four  straight  sides.  Each  of  these 
round  corners  represents  1/4  of  a  circular  cup,  and,  therefore,  the  previous  analysis 
holds  true  for  it.  On  the  other  hand,  no  lateral  compression  is  needed  to  allow  the  blank 
metal  to  flow  toward  the  die  edge  at  each  of  the  straight  sides.  Accordingly,  the  process 
in  these  zones  is  not  drawing  at  all;  it  is  just  bending  and  straightening.  For  this  rea- 
son, the  metal  in  these  zones  flows  faster  than  in  the  round  corners,  and  a  square  blank 
takes  the  form  shown  in  Figure  6.39  after  drawing.  Note  that  there  is  excess  metal  at 
each  of  the  four  round  corners,  which  impedes  the  drawing  operations  at  those  loca- 
tions. It  also  results  in  localized  higher  stresses  and  tears  almost  always  beginning  at 
one  (or  more)  of  the  corners  during  box  drawing,  as  can  be  seen  in  Figure  6.40. 

Several  variables  affect  this  complex  operation  as  well  as  the  quality  of  the  products 
obtained.  They  include  the  die  bending  radius,  the  die  corner  radius,  and  the  shape  of  the 
original  blank.  These  process  variables  have  been  investigated  by  research  workers,  and 
it  has  been  found  that  in  order  to  obtain  sound  box-shaped  cups,  it  is  very  important  to 
ensure  easy,  unobstructed  flow  of  metal  during  the  drawing  operation.  The  absence  of 
this  condition  results  in  the  initiation  of  high  tensile  stresses  in  the  vertical  walls  of  the 
box,  especially  at  the  round  corners,  and  results  in  considerable  thinning,  which  is  fol- 
lowed by  fracture.  Among  the  factors  that  can  cause  obstruction  to  the  metal  flow  are 
smaller  die  radii,  higher  reduction  ratios  (at  the  corners),  and  poor  lubrication.  These  are 
added  to  the  presence  of  excess  metal  at  the  corners,  which  causes  an  appreciable  in- 
crease in  the  transverse  compressive  stresses.  Therefore,  an  optimum  blank  shape  with- 
out excess  metal  at  the  corners  is  necessary  for  achieving  successful  drawing  operations 
of  box-shaped  cups.  A  simple  method  for  optimizing  the  shape  of  the  blank  is  shown  in 
Figure  6.41.  It  involves  printing  a  square  grid  on  the  surface  of  the  blank  and  determin- 
ing the  borders  of  the  undeformed  zone  on  the  flanges  at  each  corner  (by  observing  the 


FIGURE    6.39 

Final  shape  of  a  box- 
shaped  cup,  obtained 
by  deep  drawing  a 
square  blank 


6.1  Press  Working  Operations 


233 


FIGURE    6.40 

Tears  occurring  in  box 
drawing 


FIGURE    6.41 

Optimized  blank  shape 
for  drawing  box-shaped 
cups 


undistorted  grid)  so  that  it  can  be  taken  off  the  original  blank.  It  has  been  found  that  the 
optimum  shape  is  a  circle  with  four  cuts  corresponding  to  the  four  corners.  Also,  the 
blank-holding  force  has  been  found  to  play  a  very  important  role.  Better  products  are  ob- 
tained by  using  a  rubber-actuated  blank  holder  that  exerts  low  forces  during  the  first 
third  of  the  drawing  stroke,  followed  by  a  marked  increase  in  those  forces  during  the  rest 
of  the  drawing  stroke  to  eliminate  wrinkling  and  stretch  out  the  product. 


234 


Sheet  Metal  Working 


FIGURE    6.42 

Optimized  blank  shape 
for  drawing  cups  with 
an  irregular  cross 
section 


Line 


Original 
blank 


Cross  section  of  the 
deep-drawn  part 


The  preceding  discussion  can  be  generalized  to  include  the  drawing  of  a  cup  with 
an  irregular  cross  section.  This  can  be  achieved  by  dividing  the  perimeter  into  straight 
sides  and  circular  arcs.  Professor  Kurt  Lange  and  his  coworkers  (Institute  Fur  Um- 
formstechnik,  Stuttgart  Universitate)  have  developed  a  technique  for  obtaining  the  op- 
timum blank  shape  in  this  case  by  employing  the  slip-line  field  theory.  The  technique 
was  included  in  an  interactive  computer  expert  system  that  is  capable  of  giving  direct 
answers  to  any  drawing  problem.  An  optimized  blank  shape  obtained  by  that  system  is 
shown  in  Figure  6.42. 

Recent  developments  In  deep  drawing.  A  recent  development  in  deep  drawing  in- 
volves cup  drawing  without  a  blank  holder.  Cupping  of  a  thick  blank  has  been  ac- 
complished by  pushing  the  blank  through  a  die  having  a  special  profile,  as  shown  in 
Figure  6.43,  without  any  need  for  a  blank  holder.  This  process  has  the  advantages  of 
reducing  the  number  of  processing  stages,  eliminating  the  blank  holder,  and  using 
considerably  simpler  tool  construction.  A  further  advantage  is  that  the  operation  can 
be  performed  on  a  single-acting  press,  resulting  in  an  appreciable  reduction  in  the  ini- 
tial capital  cost  required. 

Another  new  development  is  the  employment  of  ultrasonics  to  aid  the  deep  draw- 
ing operation.  The  function  of  the  ultrasonic  waves  is  to  enlarge  the  die  bore  and  then 
leave  it  to  return  elastically  to  its  original  dimension  in  a  pulsating  manner.  This  re- 
duces the  friction  forces  appreciably,  resulting  in  a  marked  reduction  in  the  required 
drawing  force  and  in  a  clear  improvement  of  the  quality  of  the  drawn  cup.  In  many 
cases,  the  cup  can  be  drawn  by  the  force  exerted  by  the  human  hand  without  the  need 


FIGURE    6.43 

Drawing  cups  without  a 
blank  holder 


Blank 


3- 


6.1  Press  Working  Operations 


235 


for  any  mechanical  force-generating  device.  It  is,  therefore,  obvious  that  low-tonnage, 
high-production-rate  presses  can  be  used,  which  makes  the  process  economically  at- 
tractive. 

Defects  in  deep-drawn  parts.  These  defects  differ  in  shape  and  cause,  depending 
upon  the  prevailing  conditions  and  also  on  the  initial  dimensions  of  the  blank.  Fol- 
lowing is  a  brief  description  of  the  most  common  defects,  some  of  which  are  shown  in 
Figure  6.44: 

1.  Wrinkling.  Wrinkling  is  the  buckling  of  the  undrawn  part  of  the  blank  under  com- 
pressive stresses;  it  may  also  occur  in  the  vertical  walls  (see  Figure  6.44a  and  b).  If 
it  takes  place  on  the  punch  nose  when  drawing  a  domed  cup,  it  is  referred  to  as 
puckering. 

2.  Tearing.  Tearing,  which  always  occurs  in  the  vicinity  of  the  radius  connecting  the 
cup  bottom  and  the  wall,  is  caused  by  high  tensile  stresses  due  to  the  obstruction  of 
the  flow  of  the  metal  in  the  flange. 

3.  Earing.  Earing  is  the  formation  of  ears  at  the  free  edges  of  a  deep-drawn  cylindri- 
cal cup  (see  Figure  6.44c).  It  is  caused  by  the  anisotropy  of  the  sheet  metal.  Ears 
are  trimmed  after  a  drawing  operation,  resulting  in  a  waste  of  material. 

4.  Surface  irregularities.  Surface  irregularities  are  caused  by  nonuniform  yielding, 
like  the  orange-peel  effect  of  Luder's  lines. 

5.  Surface  marks.  Surface  marks  are  caused  by  improper  punch-die  clearance  or  poor 
lubrication.  These  include  draw  marks,  step  rings,  and  burnishing. 

Forming  Operations 

In  this  section,  we  will  discuss  the  various  forming  operations  performed  on  sheet  met- 
als— not  just  flat  sheets,  but  tubular  sheets  (i.e.,  thin-walled  tubes)  as  well.  Therefore, 
not  only  will  operations  like  embossing  and  offsetting  be  discussed,  but  also  tube 
bulging,  expanding,  and  necking  will  be  considered. 

Forming  of  sheets.  True  forming  involves  shaping  the  blank  into  a  three-dimensional 
(or  sculptured)  surface  by  sandwiching  it  between  a  punch  and  a  die.  The  strain  is  not 
uniform,  and  the  operation  is  complex.  The  nonhomogeneity  (or  complexity)  depends 
upon  the  nature  and  the  unevenness  of  the  required  shape.  Experience  and  trial  and 
error  were  employed  in  the  past  to  obtain  an  optimum  blank  shape  and  to  avoid  thin- 
ning the  blank  or  tearing. 


FIGURE    6.44 

Some  defects  occurring 
in  deep  drawing 
operations:  (a)  wrinkling 
in  the  flange; 

(b)  wrinkling  in  the  wall; 

(c)  earing 


(b) 


(c) 


236 


6      Sheet  Metal  Working 


A  printed  grid  on  the  original  blank  helps  to  detect  the  locations  of  overstraining 
where  tearing  is  expected.  It  also  helps  in  optimizing  the  shape  of  the  original  blank. 
With  recent  advances  in  computer  graphics  and  simulation  of  metal  deformation,  ra- 
tional design  of  the  blank  can  be  performed  by  the  computer,  without  any  need  for  trial 
and  error.  In  fact,  a  successful  software  package  has  been  prepared  by  the  Mechanical 
Engineering  Department  of  Michigan  Technological  University. 

Embossing  operations.  Embossing  operations  involve  localized  deflection  of  a  flat 
sheet  to  create  depressions  in  the  form  of  beads  and  offsets.  This  is  sometimes  called 
oil  canning.  Beads  and  offsets  are  usually  employed  to  add  stiffness  to  thin  sheets, 
whether  flat  or  tubular  (e.g.,  barrels),  as  well  as  for  other  functional  reasons.  A  typi- 
cal example  of  a  part  that  is  subjected  to  embossing  is  the  license  plate  of  an  auto- 
mobile. The  cross  section  of  a  bead  can  take  different  forms,  such  as  those  shown  in 
Figure  6.45.  Because  this  operation  involves  stretching  the  sheet,  the  achieved  local- 
ized percentage  elongation  within  the  bead  cross  section  must  be  lower  than  that  al- 
lowable for  the  metal  of  the  sheet.  On  the  other  hand,  Figure  6.46  shows  two  kinds 
of  offsets,  where  it  is  common  practice  to  take  the  maximum  permissible  depth  as 
three  times  the  thickness  of  the  sheet  metal. 

Rubber  forming  of  flat  sheets.  Rubber  forming  is  not  new  and  actually  dates  back  to 
the  nineteenth  century,  when  a  technique  for  shearing  and  cutting  paper  and  foil  was 
patented  by  Adolph  Delkescamp  in  1872.  Another  rubber  forming  technique,  called  the 
Guerin  process,  was  widely  used  during  World  War  II  for  forming  aircraft  panels.  It  in- 
volved employing  a  confined  rubber  pad  on  the  upper  platen  of  the  press  and  a  steel 
form  block  on  the  lower  platen,  as  shown  in  Figure  6.47a.  This  method  is  still  some- 
times used.  As  can  be  seen  in  the  figure,  when  a  block  of  elastomer  (usually  incom- 
pressible artificial  rubber)  is  confined  in  a  rigid  box,  the  only  way  it  can  flow  when  the 
punch  sinks  into  it  is  up,  thus  forming  the  blank  around  the  punch  under  uniform  pres- 
sure over  the  whole  surface.  It  is  also  common  industrial  practice  to  place  spacers  on 
the  base  of  the  metal  box  in  order  to  provide  a  relief  for  the  elastomer  block,  which,  in 
turn,  helps  to  avoid  the  initiation  of  high  localized  strains  in  the  blank  area  directly  be- 
neath the  punch.  Rubber  forming  has  real  potential  when  the  number  of  parts  required 
is  relatively  small  and  does  not  justify  designing  and  constructing  a  forming  die. 

A  modified  version  of  this  process,  called  the  hydroform  process,  involves  em- 
ploying a  pressurized  fluid  above  the  rubber  membrane,  as  shown  in  Figure  6.47b. 


FIGURE    6.45 

Different  kinds  of 
beads 


n 


1      c 


Vbead 


Flat  V  bead 


Round  bead 


FIGURE    6.46 

Offsetting  operations 


u 


Interior  offset 


Edge  offset 


6.1  Press  Working  Operations 


237 


FIGURE    6.47 

Rubber  forming  of  flat 
sheets:  (a)  conventional 
rubber  forming; 
(b)  hydroform  process 


Rubber 


Container 


(a) 


Rubber 


Sheet 
metal 


Forming 
punch 


(b) 


This  is  similar  in  effect  to  drawing  the  cup  into  a  high-pressure  container,  as  previously 
mentioned.  Therefore,  percentage  reductions  higher  than  those  obtained  in  conven- 
tional deep  drawing  can  be  achieved. 

Forming  of  tubular  sheets.  Figure  6.48a  through  d  indicates  tubular  parts  after  they 
were  subjected  to  beading,  flattening,  expanding,  and  necking  operations,  respectively. 
Tube  bulging  is  another  forming  operation,  in  which  the  diameter  of  the  tube,  in 
its  middle  part,  is  expanded  and  then  restrained  by  a  split  die  and  forced  to  conform 
to  the  details  of  the  internal  surface  of  the  die.  This  can  be  achieved  by  internal  hy- 
draulic pressure  or  by  employing  an  elastomer  (polyurethane)  rod  as  the  pressure- 
transmitting  medium,  causing  expansion  of  the  tube.  A  schematic  of  this  operation  is 


FIGURE    6.48 

Different  tubular  parts 
after  forming 
operations:  (a)  beading; 

(b)  flattening; 

(c)  expanding; 

(d)  necking 


(a) 


(b) 


-^7777. 


\\\\\K 


(d) 


238 


6      Sheet  Metal  Working 


FIGURE    6.49 

The  tube-bulging 
operation  with  an 
elastomer  rod 


.Punch 


Die  holder 
(ring) 


Punch 


given  in  Figure  6.49.  At  the  beginning  of  the  operation,  the  elastomer  rod  fits  freely 
inside  the  tube  and  has  the  same  length.  Compressive  forces  are  then  applied  to  both 
the  rod  and  the  tube  simultaneously  so  that  the  tube  bulges  outward  in  the  middle 
and  the  frictional  forces  at  the  tube-rod  interface  draw  more  metal  into  the  die  space, 
thus  decreasing  the  length  of  the  tube.  The  method  of  using  a  polyurethane  rod  is 
simpler  and  cleaner,  and  there  is  no  need  for  using  oil  seals  or  complicated  tooling 
construction.  A  further  advantage  of  rubber  bulging  is  that  it  can  be  used  for  simul- 
taneous forming,  piercing,  and  shearing  of  thin  tubular  sheets. 


6.2  HIGH-ENERGY-RATE  FORMING  (HERF) 


In  HERF,  the  energy  of  deformation  is  delivered  within  a  very  short  period  of  time — 
on  the  order  of  milliseconds  or  even  microseconds.  HERF  methods  include  explosive, 
electrohydraulic,  and  electromagnetic  forming  techniques.  These  techniques  are  usu- 
ally employed  when  short-run  products  or  large  parts  are  required.  HERF  is  also  rec- 
ommended for  manufacturing  prototype  components  and  new  shapes  in  order  to  avoid 
the  unjustifiable  cost  of  dies.  Rocket  domes  and  other  aerospace  structural  panels  are 
typical  examples.  During  a  HERF  process,  the  sheet  metal  is  given  an  extremely  high 
acceleration  in  a  very  short  period  of  time  and  is  thus  formed  as  a  result  of  consuming 
its  own  kinetic  energy  to  cause  deformation. 

Explosive  Forming 

Explosive  forming  of  sheet  metal  received  some  attention  during  the  past  decade.  The 
various  explosive  forming  techniques  fall  under  one  or  the  other  of  two  basic  systems: 
confined  and  unconfined.  In  a  confined  system,  which  is  shown  in  Figure  6.50a,  a 
charge  of  low  explosives  is  detonated  and  yields  a  large  amount  of  high-pressure  gas, 
thus  forcing  the  sheet  metal  to  take  the  desired  shape.  This  system  is  mainly  used  for 


6.2  High-Energy-Rate  Forming  (HERF) 


239 


FIGURE    6.50 

Explosive  forming  of 
sheet  metal: 

(a)  confined  system; 

(b)  standoff  system 


Cartridge 


Water 


Die  steel 


Tube 


(b) 


bulging  and  flaring  of  small  tubular  parts.  Its  main  disadvantage  is  the  hazard  of  die 
failure  because  of  the  high  pressure  generated. 

In  an  unconfined,  or  standoff,  system,  which  is  shown  in  Figure  6.50b,  the  charge  is 
maintained  at  a  distance  from  the  sheet  blank  (the  standoff  distance),  and  both  the  blank 
and  the  charge  are  kept  immersed  in  water.  When  the  charge  is  detonated,  shock  waves 
are  generated,  thus  forming  a  large  blank  into  the  desired  shape.  It  is  obvious  that  the  ef- 
ficiency of  the  standoff  system  is  less  than  that  of  the  confined  system  because  only  a 
portion  of  the  surface  over  which  the  shock  waves  act  is  utilized  (actually,  shock  waves 
act  in  all  directions,  forming  a  spherical  front).  However,  the  standoff  system  has  the  ad- 
vantages of  a  lower  noise  level  and  of  largely  reducing  the  hazard  of  damaging  the 
workpiece  by  particles  resulting  from  the  explosion.  In  a  simple  standoff  system,  the  dis- 
tance from  the  explosive  charge  to  the  water  surface  is  usually  taken  as  twice  the  stand- 
off distance.  The  latter  depends  upon  the  size  of  the  blank  and  is  taken  as  equal  to  D  (the 
blank  diameter)  for  D  less  than  2  feet  (60  cm)  and  is  taken  as  equal  to  0.5D  for  D  greater 
than  that.  Best  results  are  obtained  when  the  blank  is  clamped  lightly  around  its  periph- 
ery and  when  a  material  with  a  low  modulus  of  elasticity,  like  plastic,  is  used  as  a  die 
material.  This  eliminates  springback,  thus  obtaining  closer  tolerances.  A  modified  ver- 
sion of  this  method  is  illustrated  in  Figure  6.5 1,  where  a  reflector  is  used  to  collect  and 


FIGURE    6.51 

Increasing  the 
efficiency  of  explosive 
forming  by  using  a 
reflector 


Metal 
blank 


To  vacuum 


Explosive 


Reflector 


240 


Sheet  Metal  Working 


reflect  the  explosion  energy  that  does  not  fall  directly  onto  the  blank  surface.  This  leads 
to  improved  efficiency  over  the  standoff  system  because  a  smaller  amount  of  charge  is 
needed  for  the  same  job. 

Electrohydraulic  Forming 

The  basic  idea  for  the  process  of  electrohydraulic  forming,  which  has  been  known  for 
some  time,  is  based  on  discharging  a  large  amount  of  electrical  energy  across  a  small 
gap  between  two  electrodes  immersed  in  water,  as  shown  in  Figure  6.52.  The  high- 
amperage  current  resulting  from  suddenly  discharging  the  electrical  energy  from  the 
condensers  melts  the  thin  wire  between  the  electrodes  and  generates  a  shock  wave. 
The  shock  wave  lasts  for  a  few  microseconds;  it  travels  through  water  to  hit  the 
blank  and  forces  it  to  take  the  shape  of  the  die  cavity.  The  use  of  a  thin  wire  between 
the  electrodes  has  the  advantages  of  initiating  and  guiding  the  path  of  the  spark,  en- 
abling the  use  of  nonconductive  liquids;  also,  the  wire  can  be  shaped  to  suit  the 
geometry  of  the  required  product.  The  method  is  also  safer  than  explosive  forming 
and  can  be  used  for  simultaneous  operations  like  piercing  and  bulging.  Nevertheless, 
it  is  not  suitable  for  continuous  production  runs  because  the  wire  has  to  be  replaced 
after  each  operation.  Moreover,  the  level  of  energy  generated  is  lower  than  that  of  ex- 
plosive forming.  Therefore,  the  products  are  generally  smaller  than  those  produced 
by  explosive  forming. 

Electromagnetic  Forming 

Electromagnetic  forming  is  another  technique  based  on  the  sudden  discharge  of  elec- 
trical energy.  As  we  know  from  electricity  and  magnetism  in  physics,  when  an  electric 
current  passes  through  a  coil,  it  initiates  a  magnetic  field  whose  magnitude  is  a  func- 
tion of  the  current.  We  also  know  that  when  a  magnetic  field  is  interrupted  by  a  con- 
ductive material  (workpiece),  a  current  is  induced  in  that  material  that  is  proportional 
to  the  rate  of  change  of  the  flux.  This  is  called  eddy  current  and  produces  its  own  mag- 
netic field  that  opposes  the  initial  one.  As  a  result,  repulsive  forces  between  the  coil 
and  the  workpiece  force  the  workpiece  to  conform  to  the  die  cavity.  This  technique  can 
be  used  to  form  flat  as  well  as  tubular  sheets.  As  can  be  seen  in  Figure  6.53,  it  is  em- 
ployed in  expanding  as  well  as  compressing  tubes.  It  has  proven  to  be  very  effective 
when  forming  relatively  thin  materials. 


FIGURE    6.52 

Electrohydraulic  forming 


Switch 


Charger 


IN-h. , r 

HH    HH    HH 


Capacitor 
bank 


Electrodes 


6.3  Spinning  of  Sheet  Metal 


241 


FIGURE    6.53 

Examples  of 
electromagnetic 
forming  of  tubes 


Mandrel 


PINNING  OF  SHEET  METAL 


Spinning  is  the  forming  of  axisymmetric  hollow  shells  over  a  rotating-form  mandrel  by 
using  special  rollers.  Generally,  the  shapes  produced  by  spinning  can  also  be  manu- 
factured by  drawing,  compressing,  or  flanging.  However,  spinning  is  usually  used  for 
forming  large  parts  that  require  very  large  drawing  presses  or  when  there  is  a  diversity 
in  the  products  (i.e.,  when  various  shapes  are  needed  but  only  a  small  number  of  each 
shape  is  required). 

A  schematic  of  the  spinning  operation  is  shown  in  Figure  6.54.  At  the  beginning, 
the  semifinished  product  (circular  blank)  is  pushed  by  the  tail  stock  against  the  front  of 
the  form  mandrel  (usually  a  wooden  one)  that  is  fixed  on  the  rotating  faceplate  of  the 
spinning  machine  (like  a  lathe).  A  pressing  tool  is  pushed  by  the  operator  onto  the  ex- 
ternal surface  of  the  blank.  The  blank  slips  under  the  pressing  devices,  which  causes 
localized  deformation.  Finally,  the  blank  takes  the  exact  shape  of  the  form  mandrel. 
This  technique  can  also  be  used  to  obtain  hollow  products  with  a  diameter  at  the  end 
(neck)  smaller  than  that  at  the  middle.  In  this  case,  it  is  necessary  to  use  a  collapsible- 
form  mandrel,  which  is  composed  of  individual  smaller  parts  that  can  be  extracted 
from  the  neck  of  the  final  product  after  the  process  is  completed.  Figure  6.55  shows  a 
group  of  parts  produced  by  spinning. 


FIGURE    6.54 

A  schematic  of  the 
spinning  operation 


242 


Sheet  Metal  Working 


FIGURE    6.55 

A  group  of  parts 
produced  by  spinning 


A  modified  version  of  this  method  involves  replacing  the  operator  by  a  numeri- 
cally controlled  (NC)  tool.  Auxiliary  operations,  like  removing  the  excess  metal,  are 
also  carried  out  on  the  same  machine.  Better  surface  quality  and  more  uniform  thick- 
ness are  the  advantages  of  NC  spinning  over  the  conventional  techniques. 


Review  Questions 


)V 


1.  What  main  design  feature  characterizes  sheet 
metal  products? 

2.  List  some  of  the  advantages  of  press  working 
sheet  metals. 

3.  When  are  sheet  metals  formed  in  their  hot 
state?  Give  examples. 

4.  What  are  the  two  main  groups  of  press  working 
operations? 

5.  What  main  condition  must  be  fulfilled  so  that 
cutting  of  sheet  metal  (and  not  any  other  opera- 
tion) takes  place? 

6.  Use  sketches  to  explain  why  the  angle  of  incli- 
nation of  the  upper  blade  of  guillotine  shears 
must  not  exceed  15°. 


7.  Use  sketches  to  differentiate  between  the  fol- 
lowing operations:  shearing,  cutoff,  parting, 
blanking,  and  punching. 

8.  Why  must  attention  be  given  to  careful  layout 
of  blanks  on  a  sheet  metal  strip? 

9.  Describe  a  perforating  operation. 

10.  What  does  an  edge  of  a  blank  usually  look  like? 
Draw  a  sketch. 

11.  What  is  meant  by  the  percentage  penetration? 

12.  What  does  an  edge  of  a  blank  look  like  when 
the  punch-die  clearance  is  too  large? 

13.  What  does  an  edge  of  a  blank  look  like  when 
the  punch-die  clearance  is  too  tight? 


Chapter  6  Review  Questions 


243 


14.  When  are  punches  sheared  and  why? 

15.  When  are  dies  sheared  and  why? 

16.  In  what  aspect  is  fine  blanking  different  from 
conventional  blanking? 

17.  Use  sketches  to  explain  each  of  the  following 
operations:  shaving,  piercing,  and  cropping. 

18.  Can  a  drop-through  die  be  used  on  any  press? 
Why  not? 

19.  What  is  the  function  of  a  stripper  plate? 

20.  List  two  types  of  die  constructions  for  blanking 
operations. 

21.  How  can  a  washer  be  produced  in  a  single 
stroke? 

22.  What  condition  must  be  fulfilled  so  that  bend- 
ing of  sheet  metal  takes  place? 

23.  Sketch  the  common  types  of  bending  dies. 

24.  Which  die  requires  the  minimum  force  for  the 
same  thickness  of  sheet  metal? 

25.  Where  is  tearing  expected  to  occur  and  where 
is  wrinkling  expected  to  occur  when  a  sheet 
metal  is  subjected  to  bending? 

26.  What  is  springback?  Why  does  it  occur? 

27.  List  three  methods  for  eliminating  the  effects  of 
springback. 

28.  On  what  assumption  is  blank  development 
based? 

29.  List  some  operations  that  can  be  classified  as 
bending.  Use  sketches  and  explain  design  func- 
tions of  the  products. 

30.  How  can  structural  angles  be  bent? 

31.  Explain  rotary  bending,  using  sketches,  and  list 
some  of  the  advantages  of  this  operation. 

32.  Explain  how  a  seamed  tube  can  be  produced  by 
continuous  bending. 

33.  What  are  the  disadvantages  of  seamed  tubes? 

34.  Explain  deep  drawing,  using  sketches. 

35.  What  are  the  stages  involved  in  deep  drawing  a 
circular  cup?  Explain,  using  sketches. 


36.  Indicate  the  states  of  stress  at  different  locations 
in  a  cup  toward  the  end  of  a  drawing  operation. 

37.  Where  is  thickening  expected  to  occur? 

38.  At  what  location  is  thinning  maximum?  To 
what  would  this  lead? 

39.  Why  is  a  blank  holder  sometimes  needed? 

40.  List  some  of  the  variables  affecting  the  deep 
drawing  operation. 

41.  What  is  wrinkling?  Why  does  it  occur? 

42.  Describe  an  ironing  operation. 

43.  Is  there  any  limitation  on  ironing? 

44.  Why  are  conical  cups  not  drawn  directly? 

45.  What  is  actually  taking  place  when  drawing 
domed  cups? 

46.  Is  it  feasible  to  take  any  blank  shape  for  box 
drawing  operations  and  then  trim  the  excess 
metal?  Why? 

47.  What  are  the  mechanics  of  deformation  in  the 
straight-sides  areas? 

48.  What  is  the  advantage  of  ultrasonic  deep  draw- 
ing? 

49.  How  can  plates  be  drawn  without  a  blank 
holder? 

50.  List  some  of  the  defects  experienced  in  deep- 
drawn  products. 

51.  As  a  product  designer,  how  can  you  make  use 
of  the  embossing  operation  when  designing 
sheet  metal  parts? 

52.  When  would  you  recommend  using  rubber 
forming  techniques? 

53.  What  is  meant  by  high-energy-rate  forming? 

54.  When  would  you  recommend  using  explosive 
forming? 

55.  Should  the  dies  used  in  explosive  forming  be 
made  of  a  hard  material,  like  alloy  steel,  or  a 
softer  one,  like  plastic?  Why? 

56.  What  happens  if  you  make  the  hydraulic  head 
very  small  in  explosive  forming? 


244 


6      Sheet  Metal  Working 


57.  What  are  the  advantages  of  electrohydraulic 
forming?  What  are  the  disadvantages? 

58.  Use  a  sketch  to  explain  the  electromagnetic 
forming  operation. 


59.  Describe  spinning.  When  is  it  recommended? 

60.  Can  products  with  a  diameter  at  the  neck 
smaller  than  at  the  middle  be  produced  by  spin- 
ning? How? 


Problems 


o. 


1.  The  blank  shown  in  Figure  6.56  is  to  be  pro- 
duced from  a  sheet  metal  strip  0.0625  inch  (1.6 
mm)  in  thickness.  Material  is  low-carbon  steel 
AISI  1020.  Estimate  the  required  blanking  force. 

2.  The  products  shown  in  Figure  6.57a,  b,  and  c  are 
produced  by  bending.  Obtain  the  length  of  the 
original  blank  to  the  nearest  0.01  inch  (0.25 
mm).  Take  /  as  0.0625  inch  (1.6  mm). 

3.  A  cup  is  drawn  from  a  sheet  of  1020  steel.  The 
thickness  is  0.03  inch  (0.8  mm),  and  the  inner  di- 


ameter is  1  inch  (25  mm).  Estimate  the  maxi- 
mum force  required  for  drawing.  If  the  material 
is  aluminum,  what  would  the  force  be? 

A  cup  with  a  height  of  0.75  inch  (18.75  mm)  and 
an  inner  diameter  of  1  inch  (25  mm)  is  to  be 
drawn  from  a  steel  strip  0.0625  inch  ( 1 .6  mm)  in 
thickness.  Plan  for  the  drawing  process  by  carry- 
ing out  blank  development,  determining  the 
number  of  drawings,  and  looking  at  the  draw 
severity  analysis. 


FIGURE    6.56 

The  blank  shape 
required  in  Problem  1 


1.5  in. 
(37.5  mm) 


R  =  0.25  in. 
(6  mm) 


Chapter  6  Design  Example 


245 


FIGURE    6.57 

Products  produced  by 
bending  in  Problem  2 


R  =  0.5  in. 
(12.5  mm) 

R  =0.75  in. 

18.75  mm) 


(c) 


Design  Example 


m 


PROBLEM 

Design  a  simple  wrench  to  loosen  (or  tighten)  a  1/2-inch  (12.5-mm)  nut  (or  bolt  head). 
The  1/2  inch  (12.5  mm)  measures  across  the  nut  flats.  The  torque  is  1  lb  ft  (1.356 
Nm),  and  50,000  pieces  are  required  annually.  The  wrench  is  to  be  produced  by  press 
working. 

Solution 

A  suitable  method  for  production  is  fine  blanking  as  there  will  be  no  need  for  any  fur- 
ther machining  operations.  We  cannot  select  a  steel  that  has  a  high  carbon  content  be- 
cause it  will  create  problems  during  the  fine-blanking  operation.  An  appropriate  choice 
is  AISI  1035  CD  steel.  The  dimensions  of  the  wrench  are  the  same  as  those  given  in 
the  examples  on  forging  and  casting,  although  the  tolerances  can  be  kept  much  tighter. 
A  detailed  design  is  given  in  Figure  6.58. 


246 


Sheet  Metal  Working 


FIGURE    6.58 

Detailed  design  of  a 
wrench  produced  by 
stamping 


0.75  inch 


Now  it  is  time  to  check  the  maximum  tensile  stress  due  to  bending: 

/  =  -±-bh3  =  ^(0.25)(0.75)3  =  0.10546  x  10"2  in.4 

q  =  Afr  =  l  x  12  x  0.375     42851b/in2 
/       0.10546  x  10"2 
It  is  less  than  the  allowable  stress  for  1035  steel,  which  is  about  20,000  lb/in. 


wm 


Design  Projects 


1.  A  pulley  (for  a  V-belt)  that  has  4-inch  (100-mm)  outer  diameter  and  is  mounted  on 
a  shaft  that  is  3/4  inch  (19  mm)  in  diameter  was  manufactured  by  casting.  The 
process  was  slow,  and  the  rejects  formed  a  noticeable  percentage  of  the  production. 
As  a  product  designer,  you  are  required  to  redesign  this  pulley  so  that  it  can  be  pro- 
duced by  sheet  metal  working  and  welding. 

2.  Design  a  connecting  rod  for  a  sewing  machine  so  that  it  can  be  produced  by  sheet 
metal  working,  given  that  the  diameter  of  each  of  the  two  holes  is  0.5  inch  (12.5 
mm)  and  the  distance  between  the  centers  of  the  holes  is  4  inches  (100  mm). 

3.  If  a  connecting  rod  four  times  smaller  than  the  one  of  Design  Project  2  is  to  be  used 
in  a  little  toy,  how  would  the  design  change? 

4.  Design  a  table  for  the  machine  shop.  The  table  should  be  4  feet  in  height,  with  a 
surface  area  of  3  by  3  feet  (900  by  900  mm),  and  should  be  able  to  carry  a  load 


Chapter  6  Design  Projects 


247 


of  half  a  ton.  Assume  that  4000  pieces  are  required  annually  and  that  different 
parts  will  be  produced  by  sheet  metal  working  and  then  joined  together  by  nuts 
and  bolts. 

5.  A  trash  container  having  a  capacity  of  1  cubic  foot  (0.02833  m3)  is  to  be  designed 
for  manufacturing  by  sheet  metal  working.  Assume  that  it  is  required  to  withstand 
an  axial  compression  load  of  200  pounds  (890  N)  and  that  the  production  rate  is 
50,000  pieces  per  year.  Provide  a  detailed  design  for  this  trash  container. 

6.  A  connecting  lever  is  produced  by  forging.  The  lever  has  two  short  bosses,  each  at 
one  of  its  ends  and  each  with  a  vertical  hole  that  is  3/4  inch  (19  mm)  in  diameter. 
The  horizontal  distance  between  the  centers  of  the  holes  is  12  inches  (300  mm),  and 
the  vertical  distance  is  3  inches  (75  mm).  The  lever  during  functioning  is  subjected 
to  a  bending  moment  of  200  lb  ft  (271  Nm).  Because  of  the  high  percentage  of  re- 
jects and  low  production  rate,  this  connecting  lever  is  to  be  produced  by  sheet  metal 
working.  Provide  a  detailed  design  so  that  it  can  be  produced  by  this  manufactur- 
ing method. 


Chapter  7 


wder — 
tallurgy 


INTRODUCTION 

Powder  metallurgy  is  the  technology  of  producing  useful  components  shaped 
from  metal  powders  by  pressing  and  simultaneous  or  subsequent  heating  to 
produce  a  coherent  mass.  The  heating  operation  is  usually  performed  in  a  con- 
trolled-atmosphere  furnace  and  is  referred  to  as  sintering.  The  sintering  tem- 
perature must  be  kept  below  the  melting  point  of  the  powder  material  or  the 
melting  point  of  the  major  constituent  if  a  mixture  of  metal  powders  is  used. 
Therefore,  sintering  involves  a  solid-state  diffusion  process  that  allows  the 
compacted  powder  particles  to  bond  together  without  going  through  the  molten 
state.  This,  in  fact,  is  the  fundamental  principle  of  powder  metallurgy. 

Historical  background.  Although  powder  metallurgy  is  becoming  increasingly  im- 
portant in  modern  industry,  the  basic  techniques  of  this  process  are  very  old 
indeed.  The  ancient  Egyptians  used  a  crude  form  of  powder  metallurgy  as  early 
as  3000  b.c.  to  manufacture  iron  implements.  The  technique  involved  reducing 
the  ore  with  charcoal  to  obtain  a  spongy  mass  of  metal  that  was  formed  by  fre- 
quent heating  and  hammering  to  eject  the  slag  and  consolidate  the  iron  parti- 
cles together  into  a  mass  of  wrought  iron.  This  process  was  used  because  the 
primitive  ovens  then  available  were  not  capable  of  melting  iron.  The  same  tech- 
nique was  used  later  by  smiths  in  India  about  a.d.  300  to  manufacture  the  well- 
known  Delhi  pillar  weighing  6.5  tons.  This  method  was  superseded  when  more 
advanced  ovens  capable  of  melting  ferrous  metals  came  into  being. 

At  the  beginning  of  the  nineteenth  century,  powder  metallurgy  had  its  first 
truly  scientific  enunciation,  in  England,  when  Wallaston  published  details  of  the 


248 


7.1  Metal  Powders  249 

preparation  of  malleable  platinum.  As  had  happened  in  the  past,  Wallaston's 
technique  was  superseded  by  melting.  However,  the  need  for  the  powder  met- 
allurgy process  arose  again  to  satisfy  the  industrial  demand  for  high-melting- 
point  metals.  An  important  application  was  the  production  of  ductile  tungsten 
in  1909  for  manufacturing  electric  lamp  filaments. 

Why  powder  metallurgy?  As  a  result  of  the  development  of  furnaces  and  melt- 
ing techniques,  the  powder-consolidation  process  is  now  usually  used  when 
melting  metal  is  undesirable  or  uneconomical.  Fusion  is  not  suitable  when  it  is 
required  to  produce  parts  with  controlled,  unique  structures,  such  as  porous 
bearings,  filters,  metallic  frictional  materials,  and  cemented  carbides.  Also,  it 
has  been  found  that  powder  metallurgy  can  produce  certain  complicated 
shapes  more  economically  and  conveniently  than  other  known  manufacturing 
processes.  For  this  reason,  the  process  currently  enjoys  widespread  industrial 
application.  As  the  price  of  labor  and  the  cost  of  materials  continue  to  rise,  the 
powder-consolidation  technique  is  becoming  more  and  more  economical  be- 
cause it  eliminates  the  need  for  further  machining  operations,  offers  more  ef- 
ficient utilization  of  materials,  and  allows  components  to  be  produced  in 
massive  numbers  with  good  surface  finish  and  close  tolerances. 


ETAL  POWDERS 


The  Manufacture  of  Metal  Powders 

Different  methods  are  used  for  producing  metal  powders.  They  include  reduction  of 
metal  oxides,  atomization  of  molten  metals,  electrolytic  deposition,  thermal  decompo- 
sition of  carbonyls,  condensation  of  metal  vapor,  and  mechanical  processing  of  solid 
metals. 

Reduction.  In  reduction,  the  raw  material  is  usually  an  oxide  that  is  subjected  to  a  se- 
quence of  concentration  and  purification  operations  before  it  is  reduced.  Carbon,  car- 
bon monoxide,  and  hydrogen  are  used  as  reducing  agents.  Following  is  the  chemical 
formula  indicating  the  reaction  between  carbon  and  iron  oxide: 


2Fe,0,  +  3C — >  4Fe  +  3C02  T 


(7.1) 


Because  the  reaction  takes  place  at  a  high  temperature,  the  resulting  metal  particles 
sinter  together  and  form  sponges  that  are  subsequently  crushed  and  milled  to  a  powder 
suitable  for  consolidation.  Such  powders  have  low  apparent  densities  and  often  contain 
impurities  and  inclusions,  but  they  are  cheap.  Metal  powders  produced  by  this  method 
include  iron,  cobalt,  nickel,  tungsten,  and  molybdenum. 


250 


7      Powder  Metallurgy 


Atomization.  Atomization  is  frequently  used  for  producing  powders  from  low-melt- 
ing-point metals  such  as  tin,  lead,  zinc,  aluminum,  and  cadmium.  Iron  powder  can  also 
be  produced  by  atomization.  The  process  involves  forcing  a  molten  metal  through  a 
small  orifice  to  yield  a  stream  that  is  disintegrated  by  a  jet  of  high-pressure  fluid.  When 
compressed  gas  is  used  as  the  atomizing  medium,  the  resulting  powder  particles  will 
be  spherical.  The  reason  is  that  complete  solidification  takes  a  relatively  long  period, 
during  which  surface  tension  forces  have  the  chance  to  spheroidize  the  molten  metal 
droplets.  However,  when  water  is  used,  the  droplets  solidify  very  quickly  and  have  a 
ragged  or  irregular  form.  Figure  7.1  illustrates  the  atomization  technique. 

Electrolytic  deposition.  Electrolytic  deposition  involves  obtaining  metal  powders 
from  solutions  by  electrolysis.  Process  parameters  such  as  current  density  and  solution 
concentration  are  controlled  to  give  a  loose  deposit  instead  of  the  coherent  layer  ac- 
quired in  electroplating.  The  electrolytically  deposited  powders  are  then  carefully 


FIGURE    7.1 

Production  of  metal 
powders  by  atomization 


Stream  of 
molten  metal 


Atomized 
powder 


7.1  Metal  Powders  251 

washed,  dried,  and  annealed.  Such  powders  are  relatively  expensive,  but  their  impor- 
tant advantage  is  their  high  purity  and  freedom  from  nonmetallic  inclusions. 

Thermal  decomposition  of  carbonyls.  Nickel  and  iron  carbonyls  are  volatile  liquids 
having  low  boiling  points  of  110°F  and  227°F  (43°C  and  107°C),  respectively.  They 
decompose  at  temperatures  below  572°F  (300°C),  and  the  metal  is  precipitated  in  the 
form  of  a  very  fine  powder. 

Condensation  of  metal  vapor.  Condensation  is  employed  only  with  some  low-melt- 
ing-point metals.  For  example,  zinc  powder  can  be  obtained  directly  by  condensation 
of  the  zinc  vapor. 

Mechanical  processing  of  solid  metals.  Production  of  metal  powders  by  comminua- 
tion  of  solid  metals  is  accomplished  by  either  machining,  crushing,  milling,  or  any 
combination  of  these.  This  method  is  limited  to  the  production  of  beryllium  and  mag- 
nesium powders  because  of  the  expenses  involved. 

Properties  of  Metal  Powders 

The  particular  method  used  for  producing  a  metal  powder  controls  its  particle  and  bulk 
properties,  which,  in  turn,  affect  the  processing  characteristics  of  that  powder.  There- 
fore, comprehensive  testing  of  all  the  physical  and  chemical  properties  of  powders  is 
essential  prior  to  use  in  order  to  avoid  variations  in  the  final  properties  of  the  com- 
pacts. Following  are  the  important  characteristics  of  metal  powders. 

Chemical  composition.  In  order  to  determine  the  chemical  composition,  conventional 
chemical  analysis  is  used  in  addition  to  some  special  tests  that  are  applicable  only  to 
metal  powders,  such  as  weight  loss  after  reduction  in  a  stream  of  hydrogen,  which  is 
an  indirect  indication  of  the  amount  of  oxide  present.  For  example,  in  the  case  of  iron 
powder,  the  following  equation  is  used: 

159  7 
%  iron  oxide  =  %  weight  loss  x  — — —  (7.2) 

48 

=  %  weight  loss  x  3.33  (7.3) 

In  Equation  7.2,  the  fraction  on  the  right-hand  side  is  the  ratio  of  the  total  weight  of 
iron  oxide  to  the  weight  of  the  combined  oxygen  in  it,  or  (Fe203)/(03),  which  can  be 
calculated  by  summing  up  the  atomic  weights  of  each  element  in  the  numerator  and 
denominator. 

It  is  also  important  to  mention  that  the  percentages  of  nonmetallic  inclusions  will 
affect  the  maximum  achievable  density  of  the  compacted  powder  (i.e.,  the  full  theo- 
retical density).  For  example,  if  an  iron  powder  (density  of  iron  is  7.87  g/cnv  )  consists 
of  a  percent  Fe203,  b  percent  carbon,  and  c  percent  sulfur,  the  following  equation  can 
be  applied: 

100 

max.  achievable  density  =   ioo  -  (a  +  b  +  c)         a  b  c  ^'^ 

+ + + 

'•o/  Poxide         Pcarbon         Psulfur 


252  7      Powder  Metallurgy 


where  poxide,  pcarbon>  and  Psuifur  are  the  densities  of  oxide,  carbon,  and  sulfur,  respec- 
tively. Equation  7.4  can  also  be  used  in  calculating  the  maximum  achievable  density 
for  a  mixture  of  powders. 

Particle  shape.  The  particle  shape  is  influenced  by  the  method  of  powder  production 
and  significantly  affects  the  apparent  density  of  the  powder,  its  pressing  properties,  and 
its  sintering  ability. 

Particle  size.  The  flow  properties  and  the  apparent  density  of  a  metal  powder  are 
markedly  influenced  by  the  particle  size,  which  can  be  directly  determined  by  mea- 
surement on  a  microscope,  by  sieving,  or  by  sedimentation  tests. 

Particle-size  distribution.  The  particle-size  distribution  has  a  considerable  effect  on 
the  physical  properties  of  the  powder.  Sieve  testing  is  the  standard  method  used  for  the 
determination  of  the  particle-size  distribution  in  a  quantitative  manner.  The  apparatus 
used  involves  a  shaking  machine  on  which  a  series  of  standard  sieves  are  stacked  with 
the  coarsest  at  the  top  and  the  finest  at  the  bottom.  The  particle-size  distribution  is  ob- 
tained from  the  percentage  (by  weight)  of  the  sample  that  passes  through  one  sieve  but 
is  retained  on  the  next  finer  sieve.  These  sieves  are  defined  by  the  mesh  size,  which  in- 
dicates the  number  of  apertures  per  linear  inch.  After  the  test  is  performed,  the  results 
are  stated  in  a  suitable  form,  such  as  a  table  of  weight  percentages,  graphs  of  frequency 
distribution,  or  cumulative  oversize  and  undersize  curves  where  the  cumulative  size  is 
the  total  weight  percentage  above  or  below  a  particular  mesh  size. 

Specific  surface.  Specific  surface  is  the  total  surface  area  of  the  particles  per  unit 
weight  of  powder,  usually  expressed  in  square  centimeters  per  gram  (cm'/g).  The  spe- 
cific surface  has  a  considerable  influence  on  the  sintering  process.  The  higher  the  spe- 
cific surface,  the  higher  the  activity  during  sintering  because  the  driving  force  for 
bonding  during  the  sintering  operation  is  the  excess  energy  due  to  the  large  area  (high 
specific  surface). 

Flowability.  Flowability  is  the  ease  with  which  a  powder  will  flow  under  gravity 
through  an  orifice.  A  quantitative  expression  of  the  flowability  of  a  powder  is  its  flow 
rate,  which  is  determined  using  a  Hall  flowmeter.  As  illustrated  in  Figure  7.2,  this  ap- 
paratus involves  a  polished  conical  funnel  made  of  brass  having  a  half-cone  angle  of 
30°  and  an  orifice  of  0.125  inch  (3.175  mm).  The  funnel  is  filled  with  50  grams  of  the 
powder,  and  the  time  taken  for  the  powder  to  flow  from  the  funnel  is  determined,  the 
flow  rate  being  expressed  in  seconds.  The  flow  properties  are  dependent  mainly  upon 
the  particle  shape,  particle  size,  and  particle-size  distribution.  They  are  also  affected  by 
the  presence  of  lubricants  and  moisture.  Good  flow  properties  are  required  if  high  pro- 
duction rates  are  to  be  achieved  in  pressing  operations  because  the  die  is  filled  with 
powder  flowing  under  gravity  and  because  a  shorter  die-filling  time  necessitates  a  high 
powder-flow  rate. 

Bulk  (or  apparent)  density.  The  bulk  (or  apparent)  density  is  the  density  of  the  bulk 
of  a  powder  mass.  It  can  be  easily  determined  by  filling  a  container  of  known  volume 
with  the  powder  and  then  determining  the  weight  of  the  powder.  The  bulk  density  is 
the  quotient  of  the  powder  mass  divided  by  its  volume  and  is  usually  expressed  in 
grams  per  cubic  centimeter  (g/cm3).  The  apparent  density  is  influenced  by  the  same 


7.1  Metal  Powders 


253 


FIGURE    7.2 

A  sketch  of  the  Hall 
flowmeter 


1/8  in.  or  1/10 


factors  as  the  flowability — namely,  the  particle  configuration  and  the  particle-size  dis- 
tribution. 

Compressibility  and  compactibility.  Compressibility  and  compactibility  are  very  im- 
portant terms  that  indicate  and  describe  the  behavior  of  a  metal  powder  when  com- 
pacted in  a  die.  Compressibility  indicates  the  densification  ability  of  a  powder,  whereas 
compactibility  is  the  structural  stability  of  the  produced  as-pressed  compact  at  a  given 
pressure.  A  generalized  interpretation  of  these  terms  involves  graphs  indicating  the  as- 
pressed  density  versus  pressure  (for  compressibility)  and  the  as-pressed  strength  ver- 
sus pressure  (for  compactibility).  It  must  be  noted  that  these  two  terms  are  not 
interchangeable:  A  brittle  powder  may  have  good  compressibility  but  usually  has  a 
weak  as-pressed  compactibility. 

Sintering  ability.  Sintering  ability  is  the  ability  of  the  adjacent  surfaces  of  particles  in 
an  as-pressed  compact  to  bond  together  when  heated  during  the  sintering  operation. 
Sintering  ability  is  influenced  mainly  by  the  specific  surface  of  the  powder  used  and  is 
the  factor  responsible  for  imparting  strength  to  the  compact. 

Factors  Affecting  the  Selection 
of  Metal  Powders 

Probably  all  metallic  elements  can  be  made  in  powderous  form  by  the  previously  dis- 
cussed manufacturing  methods.  However,  the  powder  characteristics  will  differ  in  each 
case  and  will  depend  mainly  upon  the  method  of  manufacture.  The  task  of  the  manu- 
facturing engineer  is  to  select  the  type  of  powder  appropriate  for  the  required  job.  The 
decision  generally  depends  upon  the  following  factors: 

1.  Economic  considerations 

2.  Purity  demands 

3.  Desired  physical,  electrical,  or  magnetic  characteristics  of  the  compact 

These  considerations  will  be  discussed  in  a  later  section. 


254  7      Powder  Metallurgy 


7.2  POWDER  METALLURGY: 
THE  BASIC  PROCESS 


The  conventional  powder  metallurgy  process  normally  consists  of  three  operations: 
powder  blending  and  mixing,  powder  pressing,  and  compact  sintering. 

Blending  and  Mixing 

Blending  and  mixing  the  powders  properly  is  essential  for  uniformity  of  the  finished 
product.  Desired  particle-size  distribution  is  obtained  by  blending  in  advance  the  types 
of  powders  used.  These  can  be  either  elemental  powders,  including  alloying  powders 
to  produce  a  homogeneous  mixture  of  ingredients,  or  prealloyed  powders.  In  both 
cases,  dry  lubricants  are  added  to  the  blending  powders  before  mixing.  The  commonly 
used  lubricants  include  zinc  stearate,  lithium  stearate,  calcium  stearate,  stearic  acid, 
paraffin,  acra  wax,  and  molybdenum  disulfide.  The  amount  of  lubricant  added  usually 
ranges  between  0.5  and  1.0  percent  of  the  metal  powder  by  weight.  The  function  of  the 
lubricant  is  to  minimize  the  die  wear,  to  reduce  the  friction  that  is  initiated  between  the 
die  surface  and  powder  particles  during  the  compaction  operation,  and,  hence,  to  ob- 
tain more  even  density  distribution  throughout  the  compact.  Nevertheless,  it  is  not  rec- 
ommended that  the  just-mentioned  limits  of  the  percentage  of  lubricant  be  exceeded, 
as  this  will  result  in  extruding  the  lubricant  from  the  surfaces  of  the  particles  during 
compaction  to  fill  the  voids,  preventing  proper  densification  of  the  powder  particles 
and  impeding  the  compaction  operation. 

The  time  for  mixing  may  vary  from  a  few  minutes  to  days,  depending  upon  oper- 
ator experience  and  the  results  desired.  However,  it  is  usually  recommended  that  the 
powders  be  mixed  for  45  minutes  to  an  hour.  Overmixing  should  always  be  avoided 
because  it  may  decrease  particle  size  and  work-harden  the  particles. 


Pressing 

Pressing  consists  of  filling  a  die  cavity  with  a  controlled  amount  of  blended  powder, 
applying  the  required  pressure,  and  then  ejecting  the  as-pressed  compact,  usually 
called  the  green  compact,  by  the  lower  punch.  The  pressing  operation  is  usually  per- 
formed at  room  temperature,  with  pressures  ranging  from  10  tons/in.2  (138  MPa)  to 
60  tons/in.^  (828  MPa),  depending  upon  the  material,  the  characteristics  of  the  pow- 
der used,  and  the  density  of  the  compact  to  be  achieved. 

Tooling  is  usually  made  of  hardened,  ground,  and  lapped  tool  steels.  The  final 
hardness  of  the  die  walls  that  will  come  in  contact  with  the  powder  particles  during 
compaction  should  be  around  60  Rc  in  order  to  keep  the  die  wear  minimal.  The  die 
cavity  is  designed  to  allow  a  powder  fill  about  three  times  the  volume  (or  height)  of 
the  green  compact.  The  ratio  between  the  initial  height  of  the  loose  powder  fill  and  the 
final  height  of  the  green  compact  is  called  the  compression  ratio  and  can  be  deter- 
mined from  the  following  equation: 


7.2  Powder  Metallurgy:  The  Basic  Process 


255 


compression  ratio 


height  of  loose  powder  fill 
height  of  green  compact 

density  of  green  compact 
apparent  density  of  loose  powder 


(7.5) 


When  pressure  is  first  applied  to  metal  powders,  they  will  undergo  repacking  or 
restacking  to  reduce  their  bulk  volume  and  to  attain  better  packing  density.  The  extent 
to  which  this  occurs  depends  largely  on  the  physical  characteristics  of  the  powder  par- 
ticles. The  movement  of  the  powder  particles  relative  to  one  another  will  cause  the 
oxide  films  covering  their  surfaces  to  be  rubbed  off.  These  oxide  films  will  also  col- 
lapse at  the  initial  areas  of  contact  between  particles  because  these  areas  are  small  and 
the  magnitude  of  the  localized  pressures  are,  therefore,  extremely  high.  This  leads  to 
metal-to-metal  contact  and,  consequently,  to  cold-pressure  welding  between  the  pow- 
der particles  at  the  points  of  contact.  When  the  pressure  is  further  increased,  interlock- 
ing and  plastic  deformation  of  the  particles  take  place,  extending  the  areas  of  contact 
between  the  individual  particles  and  increasing  the  strength  and  density  of  the  coher- 
ent compacted  powder.  Plasticity  of  the  metal-powder  particles  plays  a  major  role  dur- 
ing the  second  stage  of  the  pressing  operation.  As  the  compaction  pressure  increases, 
further  densification  is  increasingly  retarded  by  work-hardening  of  the  particle  mater- 
ial and  by  friction.  Figure  7.3  shows  a  typical  plot  of  the  relationship  between  the 
achieved  density  and  the  compaction  pressure.  As  can  be  seen,  the  density  first  goes  up 
at  a  high  rate,  and  then  the  rate  of  increase  in  density  decreases  with  increasing  pres- 
sure. Consequently,  it  is  very  difficult  to  achieve  the  full  density  because  prohibitive 
pressure  is  required. 

Frictional  forces  between  the  powder  and  the  die  wall  always  oppose  the  trans- 
mission of  the  applied  pressure  in  its  vicinity.  Therefore,  the  applied  pressure  diminishes 
with  depth  in  the  case  of  single-ended  pressing  (i.e.,  when  the  compaction  pressure  is 
applied  on  only  one  side).  This  is  accompanied  by  an  uneven  density  distribution 
throughout  the  compact.  The  density  always  decreases  with  increasing  distance  from 
the  pressing  punch  face.  Figure  7.4  indicates  the  variation  of  pressure  with  depth  along 


FIGURE    7.3 

A  typical  plot  of  the 
relationship  between 
achieved  density  and 
compaction  pressure 


Compaction  pressure 


256 


7      Powder  Metallurgy 


FIGURE    7.4 

The  variation  of 
pressure  with  depth 
along  the  compact 


the  compact  as  well  as  the  resulting  variation  in  density.  It  is  always  recommended  that 
the  value  of  the  length-to-diameter  ratio  of  the  compact  be  kept  lower  than  2.0  in  order 
to  avoid  considerable  density  variations. 

In  order  to  improve  pressure  transmission  and  to  obtain  more  even  density  distri- 
bution, lubricants  are  either  admixed  with  the  powder  or  applied  to  the  die  walls.  Other 
techniques  are  also  used  to  achieve  uniform  density  distribution,  such  as  compacting 
from  both  ends  and  suspending  the  die  on  springs  or  withdrawing  it  to  reduce  the  ef- 
fects of  die-wall  friction. 

During  the  pressing  of  a  metal  powder  in  a  die,  elastic  deformation  of  the  die  oc- 
curs in  radial  directions,  leading  to  bulging  of  the  die  wall.  Meanwhile,  the  compact 
deforms  both  elastically  and  plastically.  When  the  compaction  pressure  is  released,  the 
elastic  deformation  tries  to  recover.  But  because  some  of  the  compact  expansion  is  due 
to  plastic  deformation,  the  die  tightly  grips  the  compact,  which  hinders  the  die  from  re- 
turning to  its  original  shape.  Accordingly,  a  definite  load,  called  the  ejection  load,  has 
to  be  exerted  on  the  compact  in  order  to  push  it  out  of  the  die.  Figure  7.5  illustrates  the 
sequence  of  steps  in  a  pressing  operation. 

Sintering 

Sintering  involves  heating  the  green  compact  in  a  controlled-atmosphere  furnace  to  a 
temperature  that  is  slightly  below  the  melting  point  of  the  powder  metal.  When  the 
compact  is  composed  of  mixed  elemental  powders  (e.g.,  iron  and  copper),  the  sinter- 


FIGURE    7.5 

Sequence  of  steps  in  a 
pressing  operation 


p 

W 

W//, 

v/////, 

7.2  Powder  Metallurgy:  The  Basic  Process  257 

ing  temperature  will  then  have  to  be  below  the  melting  point  of  at  least  one  major  con- 
stituent. The  sintering  operation  will  result  in  the  following: 

1.  Strong  bonding  between  powder  particles 

2.  Chemical,  dimensional,  or  phase  changes 

3.  Alloying,  in  the  case  of  mixed  elemental  powders 

Such  effects  of  the  sintering  operation  are  influenced  by  process  variables  such  as  sin- 
tering temperature,  time,  and  atmosphere. 

The  amount,  size,  shape,  and  even  nature  of  the  pores  are  changed  during  sin- 
tering. There  are  two  kinds  of  porosity:  open,  or  interconnected,  porosity  (connected 
to  the  compact  surface)  and  closed,  or  isolated,  porosity.  In  a  green  compact,  most 
of  the  porosity  is  interconnected  and  is  characterized  by  extremely  irregular  pores. 
After  sintering,  interconnected  porosity  becomes  isolated,  and  pore  spheroidization 
takes  place  because  of  the  surface  tension  forces.  Also,  the  oxide  films  covering  the 
particle  surfaces  of  a  green  compact  can  be  reduced  by  using  the  appropriate  sin- 
tering atmosphere. 

The  most  important  atmospheres  used  in  industrial  sintering  are  carbon  monoxide, 
hydrogen,  and  cracked  ammonia.  The  latter  is  commonly  used  and  is  obtained  by  cat- 
alytic dissociation  of  ammonia,  which  gives  a  gas  consisting  of  25  percent  nitrogen 
and  75  percent  hydrogen  by  volume.  Inert  gases  like  argon  and  helium  are  occasion- 
ally used  as  sintering  atmospheres,  but  cost  is  a  decisive  factor  in  limiting  their  use. 
Vacuum  sintering  is  also  finding  some  industrial  application  in  recent  years;  neverthe- 
less, the  production  rate  is  the  main  limitation  of  this  method. 

There  are  two  main  types  of  sintering  furnaces:  continuous  and  batch-operated. 
In  continuous  furnaces,  the  charge  is  usually  conveyed  through  the  furnace  on  mesh 
belts.  These  furnaces  are  made  in  the  form  of  tunnels  or  long  tubes  having  a  diame- 
ter of  not  more  than  18  inches  (45  cm).  Heating  elements  are  arranged  to  provide 
two  heating  zones:  a  relatively  low-temperature  zone,  called  a  dewaxing  zone,  in 
which  lubricants  are  removed  so  that  they  will  not  cause  harmful  reactions  in  the 
next  zone,  and  a  uniform  heating  zone,  which  has  the  required  high  temperature 
where  sintering  actually  takes  place.  A  third  zone  of  the  furnace  tube  is  surrounded 
by  cooling  coils  in  order  to  cool  the  compacts  to  ambient  temperature  in  the  con- 
trolled atmosphere  of  the  furnace,  thus  avoiding  oxidation  of  the  compacts.  Flame 
curtains  (burning  gases  like  hydrogen)  are  provided  at  both  ends  of  the  furnace  tube 
to  prevent  air  from  entering  into  the  furnace.  Figure  7.6  is  a  sketch  of  a  continuous 
sintering  furnace.  This  type  of  furnace  is  suitable  for  mass  production  because  of  its 
low  sintering  cost  per  piece  and  its  ability  to  give  more  consistent  products.  When 
small  quantities  of  compacts  must  be  sintered,  however,  batch-operated  furnaces  are 
used.  These  furnaces  (e.g.,  vacuum  furnaces)  are  also  more  suitable  when  high-purity 
products  are  required. 

The  sintering  time  varies  with  the  metal  powder  and  ranges  between  30  minutes 
and  several  hours.  However,  40  minutes  to  an  hour  is  the  most  commonly  used  sinter- 
ing time  in  industry. 


258 


Powder  Metallurgy 


FIGURE    7.6 

A  sketch  of  a  continuous  sintering  furnace 

Temperature   , 


£\ 


Flame 
curtain 


/@i 


Uniform  heating  zone 


Dewaxing 
zone 


Cooling 
zone 


Flame 
curtain 


W///////^^^^^ 


PERATIONAL  FLOWCHART 


Because  of  the  wide  variety  of  powder  metallurgy  operations,  it  may  be  difficult  for 
a  person  who  is  not  familiar  with  this  process  to  pursue  the  proper  sequence  of  op- 
erations. The  flowchart  in  Figure  7.7  is  intended  to  clearly  show  the  relationship  be- 
tween the  various  powder  metallurgy  operations  (which  will  be  discussed  later)  and 
to  give  a  bird's-eye  view  of  the  flow  of  material  to  yield  the  final  required  product. 
Nevertheless,  it  must  be  remembered  that  there  are  exceptions  and  that  some  opera- 
tions cannot  be  shown  on  the  flowchart  because  they  would  make  it  overly  detailed 
and  complicated. 


7.4  ALTERNATIVE  CONSOLIDATION 
TECHNIQUES 


There  are  many  techniques  of  consolidating  metal  powders.  They  are  classified,  as 
shown  in  Figure  7.8,  into  two  main  groups:  pressureless  and  pressure  forming.  The 
pressureless  methods  are  those  in  which  no  external  pressure  is  required.  This  group 
includes  loose  sintering,  slip  casting,  and  slurry  casting.  The,  pressure  forming  methods 
include  conventional  compaction,  vibratory  compaction,  powder  extrusion,  powder 


7.4  Alternative  Consolidation  Techniques 


259 


FIGURE    7.7 

A  flowchart  showing  the  relationship  between  the  various  powder  metallurgy  operations 


Metal  powders 


Mixing  and  blending 


Conventional 
die  pressing 


HERF 
compaction 


Cold 
isostatic 
pressing 


Powder 
rolling 


Vibratory 
compaction 


Alternative 

consolidation 

technique 


Slip 
casting 


Powder 
extrusion 


Hot 
pressing 


Hot 
isostatic 
pressing 


Sintering 


Secondary 
processing 


Finishing 
operations 


Finished  P/M  components 


Loose 
sintering 


FIGURE    7.8 

Classification  of  the  techniques  for  consolidating  metal  powders 

Consolidation 
techniques 


Pressureless 


Pressure 
forming 


Loose  sintering 


Slip 
casting 


Conventional       Vibratory 
compaction       compaction 


Powder 
extrusion 


Powder 
rolling 


HIP 


Slurry 
casting 


CIP  Explosive  Forming 

compaction      with  binders 


260  7      Powder  Metallurgy 


rolling,  hot  and  cold  isostatic  pressing,  explosive  forming,  and  forming  with  binders. 
A  detailed  account  of  conventional  powder  metallurgy  has  been  given;  following  is  a 
brief  description  of  these  other  consolidation  techniques. 

Loose  Sintering 

Loose  sintering  is  employed  in  manufacturing  filters.  It  involves  sintering  of  loose 
metal  powder  in  molds  made  of  graphite  or  ceramic  material.  The  temperature  used  is 
similar  to  that  of  conventional  sintering,  but  the  time  involved  is  usually  longer  (two 
days  when  manufacturing  stainless  steel  filters). 

Slip  Casting 

The  application  of  slip  casting  is  usually  limited  to  the  production  of  large,  intricate 
components  made  from  refractory  metals  and  cermets  (mixtures  of  metals  and  ceram- 
ics). The  slip,  which  is  a  suspension  of  fine  powder  particles  in  a  viscous  liquid,  is 
poured  into  an  absorbent  plaster-of-paris  mold.  Both  solid  and  hollow  articles  can  be 
produced  by  this  method.  When  making  hollow  objects,  excess  slip  is  poured  out  after 
a  layer  of  metal  has  been  formed  on  the  mold  surface. 

Slurry  Casting 

Slurry  casting  is  very  similar  to  slip  casting,  except  that  the  mixture  takes  the  form  of 
a  slurry  and  binders  are  usually  added.  Also,  because  the  slurry  contains  less  water, 
nonabsorbent  molds  can  be  used. 

Vibratory  Compaction 

Vibratory  compaction  involves  superimposing  mechanical  vibration  on  the  pressing 
load  during  the  compaction  operation.  The  advantages  of  this  process  include  the  con- 
siderable reduction  in  the  pressure  required  and  the  ability  to  compact  brittle  particles 
that  cannot  be  pressed  by  conventional  techniques  because  the  high  compaction  load 
required  would  result  in  fragmentation  rather  than  consolidation  of  the  powder  parti- 
cles. The  main  application  involves  the  consolidation  of  stainless  steel  and  uranium 
oxide  powders  for  nuclear  fuel  elements. 

Isostatic  Pressing 

In  isostatic  pressing  (IP),  equal  all-around  pressure  is  applied  directly  to  the  powder 
mass  via  a  pressurized  fluid.  Accordingly,  die-wall  friction  is  completely  eliminated, 
which  explains  the  potential  of  the  process  to  produce  large,  dense  parts  having  uni- 
form density  distribution.  The  process  can  be  performed  at  room  temperature  (cold  iso- 
static pressing)  or  can  be  carried  out  at  elevated  temperatures  (hot  isostatic  pressing). 
In  cold  isostatic  pressing  (CIP),  a  flexible  envelope  (usually  made  of  rubber  or 
polymers)  that  has  the  required  shape  is  filled  with  the  packed  powder.  The  envelope 
is  then  sealed  and  placed  into  a  chamber  that  is,  in  turn,  closed  and  pressurized  to  con- 


7.4  Alternative  Consolidation  Techniques 


261 


FIGURE    7.9 

The  isostatic  pressing 
operation 


Pressurized 
fluid 


esS9 


Threaded 
plug 


o  o  o 
o  o  o 

o  o  o 
o  o  o 

o  o  o 
o  o  o 

o  o  o 
o  o  o 

o  o  o 
o  o  o 

o  o  o 
o  o  o 

o  o  o 
o  o  o 

o  o  o 
o  o  o 


solidate  the  powder.  The  lack  of  rigidity  of  the  flexible  envelope  is  countered  by  using 
a  mesh  or  perforated  container  as  a  support  (see  Figure  7.9).  The  main  disadvantage  of 
this  process  is  the  low  dimensional  accuracy  due  to  the  flexibility  of  the  mold. 

In  hot  isostatic  pressing  (HIP),  both  isostatic  pressing  and  sintering  are  combined. 
Powder  is  canned  in  order  to  separate  it  from  the  pressurized  fluid,  which  is  usually 
argon.  The  can  is  then  heated  in  an  autoclave,  with  pressure  applied  isostatically.  Com- 
plete densification  and  particle  bonding  occur.  The  elevated  temperature  at  which  the 
powder  is  consolidated  results  in  a  softening  of  the  particles.  For  this  reason,  the 
process  is  used  to  compact  hard-to-work  materials  such  as  tool  steels,  beryllium, 
nickel-base  superalloys,  and  refractory  metals.  A  good  example  is  the  manufacture  of 
jet-engine  turbine  blades,  where  a  near-net  shape  is  made  from  nickel-base  superal- 
loys. A  main  disadvantage  of  this  method  is  the  long  processing  time. 

Powder  Extrusion 

Powder  extrusion  is  a  continuous  compaction  process  and  can  be  performed  hot  or 
cold.  It  is  employed  in  producing  semifinished  products  having  a  high  length-to- 
diameter  ratio,  a  geometry  that  makes  producing  them  by  conventional  powder  met- 
allurgy impossible.  The  conventional  technique  involves  packing  metal  powder  into 
a  thin  container  that  is,  in  turn,  evacuated,  sealed,  and  then  extruded.  An  emerging 
technique  involves  the  extrusion  of  suitable  mixtures  of  metal  (or  ceramic)  powders 
and  binders  such  as  dextrin  and  sugars.  It  has  been  successfully  employed  in  the 
production  of  highly  porous  materials  used  as  filters  or  fuel  cells  in  batteries. 


262  7      Powder  Metallurgy 


Powder  Rolling 


Direct  powder  rolling,  or  roll  compacting,  is  another  type  of  continuous  compaction 
process.  It  is  employed  mainly  for  producing  porous  sheets  of  nonferrous  powders  like 
copper  and  nickel.  This  process  involves  feeding  the  metal  powder  into  the  gap  be- 
tween the  two  rolls  of  a  simple  mill,  where  it  is  squeezed  and  pushed  forward  to  form 
a  sheet  that  is  sintered  and  further  rolled  to  control  its  density  and  thickness. 

High-Energy-Rate  Compaction 

The  various  HERF  compaction  techniques  are  based  on  the  same  principle,  which  is 
the  application  of  the  compaction  energy  within  an  extremely  short  period  of  time. 
Several  methods  were  developed  for  compacting  metal  powders  at  high  speeds.  Ex- 
amples are  explosives,  high-speed  presses,  and  spark  sintering.  It  is  believed  that  ex- 
plosive compaction  is  suitable  only  when  the  size  of  the  compact  and  the  density 
required  cannot  be  achieved  by  the  isostatic  compaction  process.  Nevertheless,  the 
danger  of  handling  explosives  and  the  low  cycling  times  impose  serious  limitations  on 
this  technique  in  production. 

The  use  of  high-speed  presses  like  the  Dynapak  (built  by  General  Dynamics) 
and  the  Petro-Forge  (built  by  Mechanical  Engineering  Department,  Birmingham 
University,  England)  for  powder  compaction  is,  in  practicality,  an  extension  of  the 
die-pressing  technique.  These  high-speed  presses  are  particularly  advantageous  for 
pressing  hard-to-compact  powders  and  large  components. 

There  are  also  some  other  powder-consolidation  methods  that  can  be  classified  as 
high-speed  techniques.  These  include  electrodynamic  pressing,  electromagnetic  press- 
ing, and  spark  sintering.  Electrodynamic  pressing  involves  utilizing  the  high  pressure 
produced  by  the  sudden  discharge  of  electrical  energy  to  compact  powders  at  high 
speeds.  Electromagnetic  pressing  is  based  upon  the  phenomenon  that  a  strong  mag- 
netic field  is  generated  when  electric  current  is  suddenly  discharged  through  an  in- 
ductance. This  strong  magnetic  field  is  used  for  pressing  a  thin-walled  metallic  tube 
that  contains  the  powder.  Spark  sintering  involves  the  sudden  discharge  of  electrical 
energy  into  the  powder  mass  to  puncture  the  oxide  films  that  cover  each  individual 
powder  particle  and  to  build  up  pure  metallic  contacts  between  the  particles.  After 
about  10  seconds  of  impulsive  discharging,  the  current  is  shut  off,  and  a  pressure  of 
about  14,500  lb/in2  (100  MPa)  is  applied  to  compact  the  powder  to  the  final  required 
form. 

Injection  Molding 

Although  injection  molding  is  an  emerging  process,  it  can  be  considered  as  a  version 
of  forming  with  binders,  which  is  a  rather  old  method.  The  process  involves  injection 
molding  metal  powders  that  are  precoated  with  a  thermoplastic  polymer  into  a  part 
similar  in  shape  to  the  final  required  component  but  having  larger  dimensions.  After  re- 
moving the  polymer  by  an  organic  solvent,  the  porous  compact  is  then  sintered  for  a 
long  time  in  order  to  allow  for  volume  shrinkage  and,  consequently,  an  increase  in 
density.  The  main  advantage  of  this  process  is  that  it  offers  promise  in  the  forming  of 
intricate  shapes. 


7.5  Secondary  Consolidation  Operations  263 

Hot  Pressing 

Hot  pressing  is  a  combination  of  both  the  compaction  and  the  sintering  operations.  It 
is  basically  similar  to  the  conventional  powder  metallurgy  process,  except  that  pow- 
ders are  induction  heated  during  pressing,  and,  consequently,  a  protective  atmosphere 
is  necessary.  For  most  metal  powders,  the  temperatures  used  are  moderate  (above  re- 
crystallization  temperature),  and  dies  made  of  superalloys  are  used.  The  hot  pressing 
of  refractory  metals  (e.g.,  tungsten  and  beryllium),  however,  necessitates  the  use  of 
graphite  dies.  The  difficulties  encountered  in  this  technique  limit  its  application  to  lab- 
oratory research. 

SECONDARY  CONSOLIDATION 
OPERATIONS 

In  most  engineering  applications,  the  physical  and  mechanical  properties  of  the  as- 
sintered  compact  are  adequate  enough  to  make  it  ready  for  use.  However,  secondary 
processing  is  sometimes  required  to  increase  the  density  and  enhance  the  mechani- 
cal properties  of  the  sintered  component,  thus  making  it  suitable  for  heavy-duty  en- 
gineering applications.  The  operations  involved  are  similar  to  those  used  in  forming 
fully  dense  metals,  though  certain  precautions  are  required  to  account  for  the  porous 
nature  of  the  sintered  compacts.  Following  is  a  survey  of  the  common  secondary 
operations. 

Coining  (Repressing) 

Coining  involves  the  pressing  of  a  previously  consolidated  and  sintered  compact  in 
order  to  increase  its  density.  This  operation  is  performed  at  room  temperature,  and  con- 
siderable pressures  are  thus  required.  It  is  often  possible  to  obtain  significant  improve- 
ment in  strength  not  only  because  of  the  increased  densification  but  also  because  of  the 
work-hardening  that  occurs  during  the  operation.  A  further  advantage  of  this  process  is 
that  it  can  be  employed  to  alter  shape  and  dimensions  slightly.  Repressing  is  a  special 
case  of  coining  where  no  shape  alteration  is  required. 

Extrusion,  Swaging,  or  Rolling 

Sintered  powder  compacts,  whether  in  their  cold  or  hot  state,  can  be  subjected  to  any 
forming  operation  (extrusion,  swaging,  or  rolling).  When  processing  at  elevated  tem- 
peratures, either  a  protective  atmosphere  or  canning  of  the  compacts  has  to  be  em- 
ployed. Such  techniques  are  applied  to  canned  sintered  compacts  of  refractory  metals, 
beryllium,  and  composite  materials. 

Forging  of  Powder  Preforms 

Repressing  and  coining  of  sintered  compacts  cannot  reduce  porosity  below  5  percent 
of  the  volume  of  the  compact.  Therefore,  if  porosity  is  to  be  completely  eliminated, 
hot  forging  of  powder  preforms  must  be  employed.  Sintered  powder  compacts  hav- 
ing medium  densities  (80  to  85  percent  of  the  full  theoretical  density)  are  heated. 


264 


Powder  Metallurgy 


lubricated,  and  fed  into  a  die  cavity.  The  preform  is  then  forged  with  a  single 
stroke,  as  opposed  to  conventional  forging  of  fully  dense  materials,  where  several 
blows  and  manual  transfer  of  a  billet  through  a  series  of  dies  are  required.  This  ad- 
vantage is  a  consequence  of  using  a  preform  that  has  a  shape  quite  close  to  that  of 
the  final  forged  product.  The  tooling  used  involves  a  precision  flashless  closed  die; 
therefore,  the  trimming  operation  performed  after  conventional  forging  is  eliminated. 
The  forging  of  powder  preforms  combines  the  advantages  of  both  the  basic  pow- 
der metallurgy  and  the  conventional  hot  forging  processes  while  eliminating  their 
shortcomings.  For  this  reason,  the  process  is  extensively  used  in  the  automotive  in- 
dustry in  producing  transmission  and  differential-gear  components.  Examples  of  some 
forged  powder  metallurgy  parts  are  shown  in  Figure  7.10. 


7.6  FINISHING  OPERATIONS 


Many  powder  metallurgy  products  are  ready  for  use  in  their  as-sintered  state;  however, 
finishing  processes  are  frequently  used  to  impart  some  physical  properties  or  geomet- 
rical characteristics  to  them.  Following  are  some  examples  of  the  finishing  operations 
employed  in  the  powder  metallurgy  industry. 

Sizing 

Sizing  is  the  pressing  of  a  sintered  compact  at  room  temperature  to  secure  the  desired 
shape  and  dimensions  by  correcting  distortion  and  change  in  dimensions  that  may  have 
occurred  during  the  sintering  operation.  Consequently,  this  operation  involves  only 


FIGURE    7.10 

Some  forged  powder 
metallurgy  parts 
(Courtesy  of  the  Metal 
Powder  Industries 
Federation,  Princeton, 
New  Jersey) 


(a) 


7.6  Finishing  Operations 


265 


FIGURE    7.10 
(Cont.) 

Some  forged  powder 
metallurgy  parts 
(Courtesy  of  the  Metal 
Powder  Industries 
Federation,  Princeton, 
New  Jersey) 


(b) 


limited  deformation  and  slight  density  changes  and  has  almost  no  effect  on  the  me- 
chanical properties  of  the  sintered  compact. 

Machining 

Features  like  side  holes,  slots,  or  grooves  cannot  be  formed  during  pressing,  and,  there- 
fore, either  one  or  two  machining  operations  are  required.  Because  cooling  liquids 
can  be  retained  in  the  pores,  sintered  components  should  be  machined  dry  whenever 
possible.  An  air  blast  is  usually  used  instead  of  coolants  to  remove  chips  and  cool  the 
tool. 


266  7      Powder  Metallurgy 


Oil  Impregnation 


Oil  impregnation  serves  to  provide  either  protection  against  corrosion  or  a  degree  of 
self-lubrication  or  both.  It  is  usually  carried  out  by  immersing  the  sintered  porous  com- 
pact in  hot  oil  and  then  allowing  the  oil  to  cool.  Oil  impregnation  is  mainly  used  in  the 
manufacturing  of  self-lubricating  bearings  made  of  bronze  or  iron. 

Infiltration 

Infiltration  is  permeation  of  a  porous  metal  skeleton  with  a  molten  metal  of  a  lower 
melting  point  by  capillary  action.  Infiltration  is  performed  in  order  to  fill  the  pores  and 
give  two-phase  structures  with  better  mechanical  properties.  The  widely  used  applica- 
tion of  this  process  is  the  infiltration  of  porous  iron  compacts  with  copper.  The  process 
is  then  referred  to  as  copper  infiltration  and  involves  placing  a  green  compact  of  cop- 
per under  (or  above)  the  sintered  iron  compact  and  heating  them  to  a  temperature 
above  the  melting  point  of  copper. 

Heat  Treatment 

Conventional  heat  treatment  operations  can  be  applied  to  sintered  porous  materials, 
provided  that  the  inherent  porosity  is  taken  into  consideration.  Pores  reduce  the  ther- 
mal conductivity  of  the  porous  parts  and  thus  reduce  their  rate  of  cooling.  For  sintered 
porous  steels,  this  means  poorer  hardenability.  Also,  cyanide  salts,  which  are  very  poi- 
sonous and  are  used  in  heat  treatment  salt  baths,  are  retained  in  the  pores,  resulting  in 
extreme  hazards  when  using  such  heat-treated  compacts.  Therefore,  it  is  not  advisable 
to  use  salt  baths  for  surface  treatment  of  porous  materials. 

Steam  Oxidizing 

A  protective  layer  of  magnetite  (Fe304)  can  be  achieved  by  heating  the  sintered  ferrous 
parts  and  exposing  them  to  superheated  steam.  This  will  increase  the  corrosion  resis- 
tance of  the  powder  metallurgy  parts,  especially  if  it  is  followed  by  oil  impregnation. 

Plating 

Metallic  coatings  can  be  satisfactorily  electroplated  directly  onto  high-density  and 
copper-infiltrated  sintered  compacts.  For  relatively  low-density  compacts,  electro- 
plating must  be  preceded  by  an  operation  to  seal  the  pores  and  render  the  compacts 
suitable  for  electroplating. 


7.7  POROSITY  IN  POWDER 
METALLURGY  PARTS 


The  structure  of  a  powder  metallurgy  part  consists  of  a  matrix  material  with  a  mi- 
crostructure  identical  to  that  of  a  conventional  fully  dense  metal  and  pores  that  are  a 
unique  and  controllable  feature  of  sintered  porous  materials.  For  this  reason,  powder 


7.7  Porosity  in  Powder  Metallurgy  Parts 


267 


metallurgy  materials  are  grouped  according  to  their  porosity,  which  is  quantitatively 
expressed  as  the  percentage  of  voids  in  a  part.  Those  materials  having  less  than  10  per- 
cent porosity  are  considered  to  be  high  density;  those  with  porosity  more  than  25  per- 
cent, low  density.  There  is  a  relationship  between  porosity  and  density  (both  being 
expressed  as  fractions  of  the  full  theoretical  density),  and  it  can  be  expressed  by  the 
following  equation: 

porosity  =  1  -  density  (7.6) 

As  previously  explained,  the  theoretical  density  is  not  that  of  the  fully  dense  pure 
metal  but  is  the  mean  value  of  the  densities  of  all  constituents.  These  include  not  only 
alloying  additives  but  also  impurities.  When  considering  green  densities,  the  effect  of 
lubricants  must  be  taken  into  consideration. 

Pores  are  classified  with  respect  to  their  percentage,  type,  size,  shape,  and  distrib- 
ution. The  type  can  be  either  interconnected  or  isolated.  The  volume  of  interconnected 
porosity  can  be  determined  by  measuring  the  amount  of  a  known  liquid  needed  to  sat- 
urate the  porous  powder  metallurgy  sample.  The  interconnected  porosity  is  essential 
for  successful  oil  impregnation  and  thus  is  very  important  for  the  proper  functioning 
of  self-lubricating  bearings. 

At  this  stage,  it  is  appropriate  to  differentiate  between  the  following  three  techni- 
cal terms  used  to  describe  density: 

„    ,      .  mass  of  compact  n  -, 

bulk  density  =  — ; j^ VJ> 

bulk  volume  of  compact 

mass  of  compact 

apparent  density  = ; 

rr  apparent  volume 

(/.©) 

mass  of  compact 


bulk  volume  of  compact  -  volume  of  open  pores 
mass  of  compact 


true  density  = 

true  volume 

mass  of  compact "™ 

~  bulk  volume  of  compact  -  (volume  of  open  pores 
+  volume  of  closed  pores) 

For  a  green  compact  produced  by  admixed  lubrication,  these  densities  are  mis- 
leading and  do  not  indicate  the  true  state  of  densification  due  to  the  presence  of  lubri- 
cant within  the  space  between  metal  particles.  Therefore,  the  bulk  density  must  be 
readjusted  to  give  the  true  metal  density  (TMD)  as  follows: 

, ,    „    ,               %  of  metal  n  im 

TMD  =  actual  bulk  density  x — I7-™) 


268  7      Powder  Metallurgy 


ESIGN  CONSIDERATIONS  FOR  POWDER 
METALLURGY  PARTS 


The  design  of  a  powder  metallurgy  part  and  the  design  of  the  tooling  required  to  pro- 
duce it  cannot  be  separated.  A  part  design  that  needs  either  long,  thin  tubular  punches, 
tooling  with  sharp  corners,  or  lateral  movement  of  punches  cannot  be  executed.  For 
this  reason,  the  design  of  powder  metallurgy  parts  is  often  different  from  that  of  parts 
produced  by  machining,  casting,  or  forging,  and  a  component  that  is  being  produced 
by  these  methods  has  to  be  redesigned  before  being  considered  for  manufacture  by 
powder  metallurgy.  Following  are  various  tooling  and  pressing  considerations,  some  of 
which  are  illustrated  in  Figure  7.11. 

Holes 

Holes  in  the  pressing  direction  can  be  produced  by  using  core  rods.  In  this  case,  there 
is  almost  no  limitation  on  the  general  shape  of  the  hole.  But  side  holes  and  side  slots 
are  very  difficult  to  achieve  during  pressing  and  must  be  made  by  secondary  machin- 
ing operations  (see  Figure  7.11a). 

Wall  Thickness 

It  is  not  desirable  to  have  a  wall  thickness  less  than  1/16  inch  (1.6  mm)  because  the 
punch  required  to  produce  the  thickness  will  not  be  rigid  enough  to  withstand  the  high 
stresses  encountered  during  the  pressing  operation. 

Fillets 

It  is  recommended  that  sharp  corners  be  avoided  whenever  possible.  Fillets  with  gen- 
erous radii  are  desirable,  provided  that  they  do  not  necessitate  the  use  of  punches  with 
featherlike  edges  (see  Figure  7.11b). 

Tapers 

Tapers  are  not  always  required.  However,  it  is  desirable  to  have  them  on  flange-type 
sections  and  bosses  to  facilitate  the  ejection  of  the  green  compact. 

Chamfers 

As  mentioned  earlier,  it  is  sometimes  not  desirable  to  use  radii  on  part  edges.  Cham- 
fers are  the  proper  alternative  in  preventing  burrs. 

Flanges 

A  small  flange,  or  overhang,  can  be  easily  produced.  However,  for  a  large  overhang, 
ejection  without  breaking  the  flange  is  very  difficult  (see  Figure  7.11c). 


7.8  Design  Considerations  for  Powder  Metallurgy  Parts 


269 


FIGURE    7.11 

Design  considerations 
for  powder  metallurgy 
parts:  (a)  holes; 
(b)  fillets;  (c)  flanges; 

(d)  bosses; 

(e)  undercuts 


Required         ^ 
punch 


(b) 


(c) 


\  / 


(d) 


(e) 


Bosses 

Bosses  can  be  made,  provided  that  they  are  round  in  shape  (or  almost  round)  and  that 
the  height  does  not  exceed  15  percent  of  the  overall  height  of  the  component  (see 
Figure  7. lid). 


Undercuts 

Undercuts  that  are  perpendicular  to  the  pressing  direction  cannot  be  made  because  they 
prevent  ejection  of  the  part.  If  required,  they  can  be  produced  by  a  secondary  machin- 
ing operation  (see  Figure  7.1  le). 


270  7      Powder  Metallurgy 


7.9  ADVANTAGES  AND  DISADVANTAGES 
OF  POWDER  METALLURGY 


Like  any  other  manufacturing  process,  powder  metallurgy  has  advantages  as  well  as 
disadvantages.  The  decision  about  whether  to  use  this  process  or  not  must  be  based  on 
these  factors.  The  advantages  of  powder  metallurgy  are  as  follows: 

1.  Components  can  be  produced  with  good  surface  finish  and  close  tolerances. 

2.  There  is  usually  no  need  for  subsequent  machining  or  finishing  operations. 

3.  The  process  offers  a  high  efficiency  of  material  utilization  because  it  virtually 
eliminates  scrap  loss. 

4.  Because  all  steps  of  the  process  are  simple  and  can  be  automated,  only  a  mini- 
mum of  skilled  labor  is  required. 

5.  Massive  numbers  of  components  with  intricate  shapes  can  be  produced  at  high 
rates. 

6.  The  possibility  exists  for  producing  controlled,  unique  structures  that  cannot  be 
obtained  by  any  other  process. 

The  main  disadvantages  of  the  process  are  as  follows: 

1.  Powders  are  relatively  high  in  cost  compared  with  solid  metals. 

2.  Sintering  furnaces  and  special  presses,  which  are  more  complicated  in  principle 
and  construction  than  conventional  presses,  are  necessary. 

3.  Tooling  is  very  expensive  as  several  punches  or  die  movements  are  often  used. 

4.  High  initial  capital  cost  is  involved,  and  the  process  is  generally  uneconomical 
unless  very  large  numbers  of  components  are  to  be  manufactured. 

5.  Powder  metallurgy  parts  have  inferior  mechanical  properties  due  to  porosity  (this 
does  not  apply  to  forged  powder  metallurgy  parts),  and  the  process  is  thus  primar- 
ily suitable  for  the  production  of  a  large  number  of  small,  lightly  stressed  parts. 


7.10  APPLICATIONS  OF  POWDER 
METALLURGY  PARTS 


The  applications  of  powder  metallurgy  parts  fall  into  two  main  groups.  The  first  group 
consists  of  those  applications  in  which  the  part  is  used  as  a  structural  component  that 
can  also  be  produced  by  alternative  competing  manufacturing  methods,  powder  metal- 
lurgy being  used  because  of  the  low  manufacture  cost  and  high  production  rate.  The 
second  group  includes  those  applications  in  which  the  part  usually  has  a  controlled, 
unique  structure  and  cannot  be  made  by  any  other  manufacturing  method.  Examples 
are  porous  bearings,  filters,  and  composite  materials.  Following  is  a  quick  review  of 
the  various  applications. 


7.10  Applications  of  Powder  Metallurgy  Parts 


271 


FIGURE    7.12 

Some  powder 
metallurgy  products 
(Courtesy  of  the  Metal 
Powder  Industries 
Federation,  Princeton, 
New  Jersey) 


Structural  Components 

Powder  metallurgy  used  to  be  limited  to  the  production  of  small,  lightly  stressed  parts. 
However,  with  the  recent  development  in  forging  powder  preforms,  the  process  is 
commonly  used  in  producing  high-density  components  with  superior  mechanical  prop- 
erties. Cams,  gears,  and  structural  parts  of  the  transmission  system  are  some  applica- 
tions of  the  powder  metallurgy  process  in  the  automotive,  agricultural  machinery,  and 
domestic  appliance  industries.  Figures  7.12  and  7.13  show  some  examples  of  powder 
metallurgy  products. 

The  structural  powder  metallurgy  components  are  usually  made  of  iron-base  pow- 
ders, with  or  without  additions  of  carbon,  copper,  and  other  alloying  elements  like 


FIGURE    7.13 

More  powder  metallurgy 
products  (Courtesy  of 
the  Metal  Powder 
Industries  Federation, 
Princeton,  New  Jersey) 


272  7      Powder  Metallurgy 


nickel.  Prealloyed  powders  are  also  employed,  although  they  are  less  common  than  the 
mixed  elemental  powders. 

Self-Lubricating  Bearings 

Self-lubricating  bearings  are  usually  made  by  the  conventional  die-pressing  technique, 
in  which  a  porosity  level  between  20  and  40  percent  is  achieved.  A  sizing  operation  is 
performed  for  dimensional  accuracy  and  in  order  to  obtain  smooth  surfaces.  The  bear- 
ings are  oil  impregnated  either  before  or  after  sizing.  Bronze  powders  are  used  in  the 
manufacturing  of  porous  bearings,  but  iron-base  powders  are  also  employed  to  give 
higher  strength  and  hardness. 

Filters 

In  manufacturing  filters,  the  appropriate  metal  powder  (e.g.,  bronze)  is  screened  in 
order  to  obtain  uniform  particle  size.  The  powder  is  then  poured  into  a  ceramic  or 
graphite  mold.  The  mold  is  put  into  a  sintering  furnace  at  the  appropriate  sintering 
temperature  so  that  loose  sintering  can  take  place.  The  products  must  have  generous 
tolerances,  especially  on  their  outer  diameters,  where  3  percent  is  typical. 

Friction  Materials 

Clutch  liners  and  brake  bands  are  examples  of  friction  materials.  They  are  best  manu- 
factured by  powder  metallurgy.  The  composition  includes  copper  as  a  matrix,  with  ad- 
ditions of  tin,  zinc,  lead,  and  iron.  Nonmetallic  constituents  like  graphite,  silica,  emery, 
or  asbestos  are  also  added.  The  mixture  is  then  formed  to  shape  by  cold  pressing.  After 
sintering,  some  finishing  operations  like  bending,  drilling,  and  cutting  are  usually  re- 
quired. It  must  be  noted  that  friction  materials  are  always  joined  to  a  solid  plate,  which 
gives  adequate  support  to  these  weak  parts. 

Electrical  Contact  Materials 

Electrical  contact  materials  include  two  main  kinds:  sliding  contacts  and  switching 
contacts.  It  is  not  possible  to  produce  any  of  these  contact  materials  except  by  powder 
metallurgy  as  both  involve  duplex  structures. 

Sliding  contacts  are  components  of  electrical  machinery  employed  when  current  is 
transferred  between  sliding  parts  (e.g.,  brushes  in  electric  motors).  The  two  main  char- 
acteristics needed  are  a  low  coefficient  of  friction  and  good  electrical  conductivity. 
Compacts  of  mixtures  of  graphite  and  metal  powder  can  fulfill  such  conditions.  Pow- 
ders of  metals  having  high  electrical  conductivity,  such  as  brass,  copper,  or  silver,  are 
used.  These  graphite-metal  contacts  are  produced  by  conventional  pressing  and  sinter- 
ing processes. 

Switching  contacts  are  used  in  high-power  circuit  breakers.  The  three  characteris- 
tics needed  are  good  electrical  conductivity,  resistance  to  mechanical  wear,  and  less 
tendency  of  the  contact  surfaces  to  weld  together.  A  combination  of  copper,  silver,  and 
a  refractory  metal  like  tungsten  provides  the  required  characteristics.  These  contacts 


7.10  Applications  of  Powder  Metallurgy  Parts  273 

are  produced  either  by  conventional  pressing  and  sintering  or  by  infiltrating  a  porous 
refractory  material  with  molten  copper  or  silver. 

Magnets 

Magnets  include  soft  magnets  and  permanent  magnets.  Soft  magnets  are  used  in  dc 
motors  or  generators  as  armatures,  as  well  as  in  measuring  instruments.  They  are  made 
of  iron,  iron-silicon,  and  iron-nickel  alloys.  Electrolytic  iron  powder  is  usually  used 
because  of  its  high  purity  and  its  good  compressibility,  which  allows  the  high  compact 
densities  required  for  maximum  permeability  to  be  attained. 

Permanent  magnets  produced  by  powder  metallurgy  have  the  commonly  known 
name  Alnico.  This  alloy  consists  mainly  of  nickel  (30  percent),  aluminum  (12  percent), 
and  iron  (58  percent)  and  possesses  outstanding  permanent  magnetic  properties.  Some 
other  additives  are  often  used,  including  cobalt,  copper,  titanium,  and  niobium. 

Cores 

The  cores  produced  by  powder  metallurgy  are  used  with  ac  high-frequency  inductors 
in  wireless  communication  systems.  Such  cores  must  possess  high  constant  perme- 
ability for  various  frequencies  as  well  as  high  electrical  resistivity.  Carbonyl  iron  pow- 
der is  mixed  with  a  binder  containing  insulators  (to  insulate  the  powder  particles  from 
one  another  and  thus  increase  electrical  resistivity)  and  then  compacted  using  ex- 
tremely high  pressures,  followed  by  sintering. 

Powder  Metallurgy  Tool  Steels 

The  production  of  tool  steels  by  powder  metallurgy  eliminates  the  defects  encountered 
in  conventionally  produced  tool  steels — namely,  segregation  and  uneven  distribution 
of  carbides.  Such  defects  create  problems  during  tool  fabrication  and  result  in  shorter 
tool  life.  The  technique  used  involves  compacting  prealloyed  tool-steel  powders  by  hot 
isostatic  pressing  to  obtain  preforms  that  are  further  processed  by  hot  working. 

Superalloys 

Superalloys  are  nickel-  and  cobalt-base  alloys,  which  exhibit  high  strength  at  elevated 
temperatures.  They  are  advantageous  in  manufacturing  jet-engine  parts  like  turbine 
blades.  The  techniques  used  in  consolidating  these  powders  include  HIP,  hot  extrusion, 
and  powder  metallurgical  forging. 

Refractory  Metals 

The  word  refractory  means  "difficult  to  fuse."  Therefore,  metals  with  high  melting 
points  are  considered  to  be  refractory  metals.  These  basically  include  four  metals: 
tungsten,  molybdenum,  tantalum,  and  niobium.  Some  other  metals  can  also  be  con- 
sidered to  belong  to  this  group.  Examples  are  platinum,  zirconium,  thorium,  and  tita- 
nium. Refractory  metals,  as  well  as  their  alloys,  are  best  fabricated  by  powder 
metallurgy.  The  technique  used  usually  involves  pressing  and  sintering,  followed  by 


274 


Powder  Metallurgy 


working  at  high  temperatures.  The  applications  are  not  limited  to  incandescent  lamp 
filaments  and  heating  elements;  they  also  include  space  technology  materials,  the 
heavy  metal  used  in  radioactive  shielding,  and  cores  for  armor-piercing  projectiles. 
Titanium  is  gaining  an  expanding  role  in  the  aerospace  industry  because  of  its  excel- 
lent strength-to-specific-weight  ratio  and  its  good  fatigue  and  corrosion  resistance. 

Cemented  Carbides 

Cemented  carbides  are  typical  composite  materials  that  possess  the  superior  properties 
of  both  constituents.  Cemented  carbides  consist  of  hard  wear-resistant  particles  of 
tungsten  or  titanium  carbides  embedded  in  a  tough  strong  matrix  of  cobalt  or  steel. 
They  are  mainly  used  as  cutting  and  forming  tools;  however,  there  are  other  applica- 
tions, including  gages,  guides,  rock  drills,  and  armor-piercing  projectiles.  They  possess 
excellent  red  hardness  and  have  an  extremely  long  service  life  as  tools.  Cemented  car- 
bides are  manufactured  by  ball-milling  carbides  with  fine  cobalt  (or  iron)  powder,  fol- 
lowed by  mixing  with  a  lubricant  and  pressing.  The  green  compact  is  then  presintered 
at  a  low  temperature,  machined  to  the  required  shape,  and  sintered  at  an  elevated  tem- 
perature. A  new  dimension  in  cemented  carbides  is  Ferro-Tic,  involving  titanium  car- 
bide particles  embedded  in  a  steel  matrix.  This  material  can  be  heat  treated  and  thus 
can  be  easily  machined  or  shaped. 


Review  Questions 


,v 


1.  Define  each  of  the  following  technical  terms: 

a.  compressibility 

b.  compactibility 

c.  green  density 

d.  impregnation 

e.  infiltration 

f.  flowability 

g.  particle-size  distribution 

2.  List  five  advantages  of  the  powder  metallurgy 
process. 

3.  List  four  disadvantages  of  the  powder  metal- 
lurgy process. 

4.  What  are   the   important  characteristics  of  a 
metal  powder? 

5.  Describe  three  methods  for  producing  metal 
powders. 


8. 


Explain  briefly  the  mechanics  of  pressing. 

Why  are  lubricants  added  to  metal  powders  be- 
fore pressing? 

Is  it  possible  to  eliminate  all  voids  by  conven- 
tional die  pressing?  Why? 

9.  Explain  briefly  the  mechanics  of  sintering. 

10.  Why  is  it  necessary  to  have  controlled  atmos- 
pheres for  sintering  furnaces? 

Explain  why  it  is  not  possible  to  use  the  con- 
ventional pressing  techniques  as  a  substitute  for 
each  of  the  following  operations:  isostatic 
pressing,  slip  casting,  HERF  compaction. 

Differentiate  between  the  following:  coining, 
repressing,  sizing. 

How  is  copper  infiltration  accomplished  and 
what  are  its  advantages? 


11 


12 


13 


Chapter  7  Problems 


275 


14.  Can  powder  metallurgical  forging  be  replaced 
by  conventional  forging?  Why? 

15.  How  can  machining  of  some  powder  metal- 
lurgy components  be  inevitable? 

16.  How  is  plating  of  powder  metallurgy  compo- 
nents carried  out? 

17.  Name  five  products  that  can  only  be  produced 
by  powder  metallurgy. 


18.  Why  are  cemented  carbides  presintered? 

19.  Why  is  electrolytic  iron  powder  used  in  manu- 
facturing soft  magnets? 

20.  Discuss  four  design  limitations  in  connection 
with  powder  metallurgy  components. 


Problems 

2 


1.  Following   are   the   experimentally   determined 
characteristics  of  three  kinds  of  iron  powder: 


f 

Sponge  Iron  Powder  (1) 

\ 

Apparent 

Screen  Analysis 

Chemical 

Composition 

Density 

Row  Rate 

+70                      0% 

H2  loss 

0.2% 

-70    to  +100     1% 

C 

0.02% 

2.4  g/cm3 

35  s/50  g 

-100  to  +325  74% 

Si02 

0.2% 

-325                 25% 

P 
S 

0.015% 
0.015% 

Sponge  Iron  Powder  (II) 

Apparent 

Screen  Analysis 

Chemical 

Composition 

Density 

Flow  Rate 

+40                        2% 

H2  loss 

0.2% 

-40  to  +60        40% 

C 

0.1% 

2.4  g/cm3 

35  s/50  g 

-70  to  +100      30% 

Si02 

0.3% 

-100  to  +200  20% 

S 

0.015% 

-200                     8% 

P 

0.015% 

276 


7      Powder  Metallurgy 


Atomized  Iron  Powder 

Screen  Analysis 

Chemical 

Composition 

Apparent 
Density 

Flow  Rate 

+100 

1% 

H2  loss 

0.2% 

+140 

45% 

C 

0.01% 

2.9  g/cm3 

24  s/50  g 

+200 

25% 

+230 

15% 

+325 

10% 

-325 

4% 

Plot  the  following  for  each  powder: 

a.  Cumulative  oversize  graph 

b.  Cumulative  undersize  graph 

c.  Frequency  distribution  curve  (obtain  median 
particle  size  for  the  powder) 

d.  Histogram  of  particle-size  distribution 

The  axis  usually  indicates  the  particle  size  in  mi- 
crons and  not  mesh  size.  Use  the  following 
table: 


4.  Plot  a  graph  indicating  the  maximum  achievable 
green  density  versus  the  percentage  of  admixed 
zinc  stearate  for  atomized  iron  powder  (density 
of  zinc  stearate  is  1 . 1  g/cm  ).  What  can  you  de- 
duce from  the  curve? 

5.  Which  powder  in  Problem  1  would  fill  the  die 
cavity  faster? 


Sieve 
Microns 


40 

420 


60 
250 


70 
210 


100 
149 


240 
105 


200 

74 


230 
63 


325 

44 


Calculate  the  full  theoretical  density  for  a  com- 
pact made  of  atomized  iron  powder,  knowing 
that  the  density  of  carbon  equals  2.2  g/cm  and 
the  density  of  iron  oxide  equals  2.9  g/cm  . 

Determine  the  approximate  height  of  the  pow- 
der fill  for  each  kind  of  iron  powder  given  in 
Problem  1  if  the  green  density  of  the  compact  is 
6.8  g/cm3  and  its  height  is  2.1  cm. 


Calculate  the  maximum  achievable  green  density 
of  a  mixture  of  atomized  iron  powder  plus  1  per- 
cent zinc  stearate  and  10  percent  pure  copper 
(density  of  copper  is  8.9  g/cm3). 


Chapter  7  Design  Project 


277 


7.  Following  is  the  relationship  between  density 
and  pressure  for  atomized  iron  powder  contain- 
ing 1  percent  zinc  stearate: 


8.  A  cylindrical  compact  of  atomized  iron  powder 
plus  1  percent  zinc  stearate  had  a  green  bulk  den- 
sity of  7.0  g/cm\  a  diameter  equal  to  2  cm,  and 


Green  density,  g/cm3 
Pressure,  MN/m2 


5.35 
157.5 


6.15 
315 


6.58 
472.5 


6.75 
629.9 


6.9 
787.4 


If  it  is  required  to  manufacture  a  gear  wheel  hav- 
ing a  green  density  of  6.8  g/cm3  using  a  press 
with  a  capacity  of  1  MN,  calculate  the  diameter 
of  the  largest  gear  wheel  that  can  be  manufac- 
tured. How  can  you  produce  a  larger  gear  by 
modifying  the  design? 


a  height  equal  to  3  cm.  After  sintering,  its  bulk 
density  increased  to  7.05  g/cm3.  Calculate  its 
new  dimensions. 

9.  The  sintered  density  of  atomized  iron  compact 
containing  10  percent  copper  was  7.2  g/cm3. 
What  is  the  porosity? 


Design  Project 


1 


Figure  7.14  shows  a  part  that  is  currently  produced  by  forging  and  subsequent  ma- 
chining. Because  the  part  is  not  subjected  to  high  stresses  during  its  actual  service  con- 
ditions, the  producing  company  is  considering  the  idea  of  manufacturing  it  by  powder 
metallurgy  in  order  to  increase  the  production  rate.  Redesign  this  component  so  that  it 
can  be  manufactured  by  the  conventional  die-pressing  technique. 


FIGURE    7.14 

A  part  to  be  redesigned 
for  production  by 
powder  metallurgy 


Chapter  8 


INTRODUCTION 

Plastics,  which  are  more  correctly  called  polymers,  are  products  of  macromo- 
lecular  chemistry.  In  fact,  the  term  polymer  is  composed  of  the  two  Greek 
words  poly  and  meres,  which  mean  "many  parts."  This  is,  indeed,  an  accurate 
description  of  the  molecule  of  a  polymer,  which  is  made  up  of  a  number  of  iden- 
tical smaller  molecules  that  are  repeatedly  linked  together  to  form  a  long  chain. 
As  an  example,  consider  the  commonly  used  polymer  polyethylene,  which  is 
composed  of  many  ethylene  molecules  (C2H4)  that  are  joined  together,  as 
shown  in  Figure  8.1.  These  repeated  molecules  are  always  organic  compounds, 
and,  therefore,  carbon  usually  forms  the  backbone  of  the  chain.  The  organic 
compound  whose  molecules  are  linked  together  (like  ethylene)  is  referred  to  as 
the  monomer. 

Now,  let  us  examine  why  the  molecules  of  a  monomer  tend  to  link  together. 
We  know  from  chemistry  that  carbon  has  a  valence  of  4.  Therefore,  each  car- 
bon atom  in  an  ethylene  molecule  has  an  unsaturated  valence  bond.  Conse- 
quently, if  two  ethylene  molecules  attach,  each  to  one  side  of  a  third  molecule, 
the  valence  bonds  on  the  two  carbon  atoms  of  the  center  molecule  will  be  sat- 
isfied (see  Figure  8.1).  In  other  words,  the  molecules  of  the  monomer  tend  to 
attach  to  one  another  to  satisfy  the  valence  requirement  of  the  carbon  atoms. 

The  molecules  of  a  monomer  in  a  chain  are  strongly  bonded  together.  Nev- 
ertheless, the  long  chains  forming  the  polymer  molecules  tend  to  be  more  or 
less  amorphous  and  are  held  together  by  weaker  secondary  forces  that  are 
known  as  the  van  der  Waals  forces  (named  after  the  Dutch  physicist).  There- 


278 


8.1  Classification  of  Polymers 


279 


FIGURE    8.1 

The  molecular  chain  of 
polyethylene 


H  H 


H       ,      H 


I 


r__ 

H      I      H 


H      i      H  H 

1 I 


H  H 


H  H 


fore,  polymers  are  generally  not  as  strong  as  metals  or  ceramics.  It  is  also  ob- 
vious that  properties  of  a  polymer  such  as  the  strength,  elasticity,  and  relax- 
ation are  dependent  mainly  upon  the  shape  and  size  of  the  long  chainlike 
molecules,  as  well  as  upon  the  mutual  interaction  between  them. 

A  common,  but  not  accurate,  meaning  of  the  term  polymer  involves  syn- 
thetic organic  materials  that  are  capable  of  being  molded.  Actually,  polymers 
form  the  building  blocks  of  animal  life;  proteins,  resins,  shellac,  and  natural 
rubber  are  some  examples  of  natural  polymers  that  have  been  in  use  for  a  long 
time.  On  the  other  hand,  the  synthetic,  or  manufactured,  polymers  have  come 
into  existence  fairly  recently.  The  first  synthetic  polymer,  cellulose  nitrate  (cel- 
luloid), was  prepared  in  1869.  It  was  followed  in  1909  by  the  phenolics,  which 
were  used  as  insulating  materials  in  light  switches.  The  evolution  of  new  poly- 
mers was  accelerated  during  World  War  II  due  to  the  scarcity  of  natural  materi- 
als. Today,  there  are  thousands  of  polymers  that  find  application  in  all  aspects 
of  our  lives. 


LASSIFICATION  OF  POLYMERS 


There  are,  generally,  two  methods  for  classifying  polymers.  The  first  method  involves 
grouping  all  polymers  based  on  their  elevated-temperature  characteristics,  which  actu- 
ally dictate  the  manufacturing  method  to  be  used.  The  second  method  of  classification 
groups  polymers  into  chemical  families,  each  of  which  has  the  same  monomer.  As  an 
example,  the  ethenic  family  is  based  on  ethylene  as  the  monomer,  and  different  poly- 
mers (members  of  this  family  such  as  polyvinyl  alcohol  or  polystyrene)  can  be  made 
by  changing  substituent  groups  on  the  basic  monomer,  as  shown  in  Figure  8.2.  As  we 
will  see  later,  this  enables  us  to  study  most  polymeric  materials  by  covering  just  a  lim- 
ited number  of  families  instead  of  considering  thousands  of  polymers  individually.  But 
before  reviewing  the  commonly  used  chemical  families  of  polymers,  let  us  discuss  in 
depth  their  elevated-temperature  behavior.  Based  on  this  behavior,  polymers  can  be 
split  into  two  groups:  thermoplastics  and  thermosets. 


280 


8      Plastics 


FIGURE    8.2 

Structural  formula  of  some  polymers  of  the  ethenic  group 


H  CC 


H  H 

Polyvinyl  chloride 


H 

OH 

—  C 

-c  — 

H 

H 

Polyv 

nyl 

alcohol 

H  CH, 


H  H 

Polypropylene 


Thermoplastics 

Thermoplastics  generally  have  linear  structures,  meaning  that  their  molecules  look 
like  linear  chains  having  little  breadth  but  significant  length.  This  structure,  as  shown 
in  Figure  8.3,  is  analogous  to  a  bowl  of  spaghetti.  Bonds  between  the  various  molecu- 
lar chains  are  mainly  of  the  van  der  Waals  type  (i.e.,  secondary  forces).  Therefore,  this 
type  of  polymer  softens  by  heating  and  can  then  flow  viscously  to  take  a  desired  shape 
because  elevated  temperatures  tend  to  decrease  the  intermolecular  coherence  of  the  lin- 
ear chains.  When  the  solidified  polymers  are  reheated  and  melted  again,  they  can  be 
given  a  different  shape.  This  characteristic  enables  plastics  fabricators  to  recycle  ther- 
moplastic scrap,  thus  increasing  the  efficiency  of  raw-material  utilization. 

Usually,  a  thermoplastic  polymer  consists  of  a  mixture  of  molecular  chains  having 
different  lengths.  Therefore,  each  structure  has  a  different  melting  point,  and,  conse- 
quently, the  whole  polymer  melts,  not  at  a  definite  temperature,  but  within  a  range 
whose  limits  are  referred  to  as  the  softening  point  and  the  flow  point.  It  has  been  ob- 
served that  when  a  thermoplastic  is  given  a  shape  at  a  temperature  between  the  soft- 


FIGURE    8.3 

The  molecular  chains  of 
a  thermoplastic 
polymer 


FIGURE    8.4 

The  molecular  chains  of 
a  thermosetting 
polymer 


8.1  Classification  of  Polymers  281 

ening  and  the  flow  points,  the  intermolecular  tension  is  retained  after  the  thermoplas- 
tic cools  down.  Therefore,  if  the  part  is  reheated  to  a  temperature  above  the  softening 
point,  it  will  return  to  its  original  shape  because  of  this  intermolecular  tension.  This 
phenomenon,  which  characterizes  most  thermoplastic  polymers,  is  known  as  shaping 
memory. 

Many  thermoplastic  polymers  are  soluble  in  various  solvents.  Consequently,  any 
one  of  these  polymers  can  be  given  any  desired  shape  by  dissolving  it  into  an  appro- 
priate solvent  and  then  casting  the  viscous  solution  in  molds.  When  the  solvent  com- 
pletely evaporates,  it  leaves  the  rigid  resin  with  the  desired  shape. 

Several  chemical  families  of  polymeric  materials  can  be  categorized  as  thermo- 
plastic. These  include  the  ethenics,  the  polyamides,  the  cellulosics,  the  acetals,  and  the 
polycarbonates.  Their  characteristics,  methods  of  manufacture,  and  applications  are 
discussed  in  detail  later  in  this  chapter. 

Thermosets 

The  molecules  of  thermosets  usually  take  the  form  of  a  three-dimensional  network 
structure  that  is  mostly  cross-linked,  as  shown  in  Figure  8.4.  When  raw  (uncured) 
thermosetting  polymers  are  heated  to  elevated  temperatures,  they  are  set,  cross- 
linked,  or  polymerized.  If  reheated  after  this  curing  operation,  thermosets  will  not 
melt  again  but  will  char  or  burn.  Therefore,  for  producing  complex  shapes  of  ther- 
mosetting polymers,  powders  (or  grains)  of  the  polymers  are  subjected  to  heat  and 
pressure  until  they  are  cured  as  finished  products.  Such  polymers  are  referred  to  as 
heat-convertible  resins. 

Some  raw  thermosets  can  take  the  form  of  liquids  at  room  temperature.  When  re- 
quired, they  are  converted  into  solids  by  curing  as  a  result  of  heating  and/or  additives 
(hardeners).  This  characteristic  enables  fabricators  to  produce  parts  by  casting  mix- 
tures of  liquid  polymers  and  hardeners  into  molds.  Therefore,  these  polymers  are  re- 
ferred to  as  casting  resins. 

The  cured  thermosets  are  insoluble  in  solvents  and  do  not  soften  at  high  tempera- 
tures. Thus,  products  made  of  thermosets  can  retain  their  shape  under  combined  load 
and  high  temperatures,  conditions  that  thermoplastics  cannot  withstand. 


282  8      Plastics 


PROPERTIES  CHARACTERIZING 
PLASTICS  AND  THEIR  EFFECT 
ON  PRODUCT  DESIGN 


Properties  of  plastics  differ  significantly  from  those  of  metals,  and  they  play  a  very  im- 
portant role  in  determining  the  form  of  the  product.  In  other  words,  the  form  is  dictated 
not  only  by  the  function  but  also  by  the  properties  of  the  material  used  and  the  method 
of  manufacture,  as  we  will  see  later.  Following  is  a  discussion  of  the  effect  of  the  prop- 
erties characterizing  plastics  on  the  design  of  plastic  products. 

Mechanical  Properties 

The  mechanical  properties  of  polymers  are  significantly  inferior  to  those  of  metals. 
Strength  and  rigidity  values  for  plastics  are  very  low  compared  with  the  lowest  values 
of  these  properties  for  metals.  Therefore,  larger  sections  must  be  provided  for  plastic 
products  if  they  are  to  have  a  similar  strength  and/or  rigidity  as  metal  products.  Un- 
fortunately, these  properties  get  even  worse  when  plastic  parts  are  heated  above  mod- 
erate temperatures.  In  addition,  some  plastics  are  extremely  brittle  and  notch-sensitive. 
Accordingly,  any  stress  raisers  like  sharp  edges  or  threads  must  be  avoided  in  such 
cases. 

A  further  undesirable  characteristic  of  plastics  is  that  they  tend  to  deform  contin- 
ually under  mechanical  load  even  at  room  temperature.  This  phenomenon  is  acceler- 
ated at  higher  temperatures.  Consequently,  structural  components  made  of  plastics 
should  be  designed  based  on  their  creep  strength  rather  than  on  their  yield  strength. 
This  dictates  a  temperature  range  in  which  only  a  plastic  product  can  be  used.  It  is  ob- 
vious that  such  a  range  is  dependent  principally  upon  the  kind  of  polymer  employed. 

In  spite  of  these  limitations,  the  strength-to-weight  ratio  as  well  as  the  stiffness-to- 
weight  ratio  of  plastics  can  generally  meet  the  requirements  for  many  engineering  ap- 
plications. In  fact,  the  stiffness-to-weight  ratio  of  reinforced  polymers  is  comparable  to 
that  of  metals  like  steel  or  aluminum. 

Physical  Properties 

Three  main  physical  properties  detrimentally  affect  the  widespread  industrial  applica- 
tion of  polymers  and  are  not  shared  by  metals.  First,  plastics  usually  have  a  very  high 
coefficient  of  thermal  expansion,  which  is  about  ten  times  that  of  steel.  This  has  to  be 
taken  into  consideration  when  designing  products  involving  a  combination  of  plastics 
and  metals.  If  plastics  are  tightly  fastened  to  metals,  severe  distortion  will  occur  when- 
ever a  significant  temperature  rise  takes  place.  Second,  some  plastics  are  inflammable 
(i.e.,  not  self-extinguishing)  and  keep  burning  even  after  the  removal  of  the  heat 
source.  Third,  some  plastics  have  the  ability  to  absorb  large  amounts  of  moisture  from 
the  surrounding  atmosphere.  This  moisture  absorption  is  unfortunately  accompanied 
by  a  change  in  the  size  of  the  plastic  part.  Nylons  are  a  typical  example  of  this  kind  of 
polymer. 


8.3  Polymeric  Systems 


283 


►OLYMERIC  SYSTEMS 


This  section  surveys  the  commonly  used  polymeric  materials  and  discusses  their  man- 
ufacturing properties  and  applications.  Also  discussed  are  the  different  additives  that 
are  used  to  impart  certain  properties  to  the  various  polymers. 

Commonly  Used  Polymers 

Following  are  some  polymeric  materials  that  are  grouped  into  chemical  families  ac- 
cording to  their  common  monomer. 

Ethenic  group.  The  monomer  is  ethylene.  This  group  includes  the  following 
polymers: 

1.  Polyethylene. 


H 

H 

—  c  — 

—  c  — 

H 

H 

The  properties  of  polyethylene  depend  upon  factors  like  degree  of  crystallinity, 
density,  molecular  weight,  and  molecular  weight  distribution.  This  thermoplastic 
polymer  is  characterized  by  its  chemical  resistance  to  solvents,  acids,  and  alkalies,  as 
well  as  by  its  toughness  and  good  wear  properties.  Polyethylenes  also  have  the  ad- 
vantage of  being  adaptable  to  many  processing  techniques,  such  as  injection  mold- 
ing, blow  molding,  pipe  extrusion,  wire  and  cable  extrusion,  and  rotational  molding. 

The  applications  of  polyethylene  are  dependent  upon  the  properties,  which,  in 
turn,  depend  upon  the  density  and  molecular  weight.  Low-density  polyethylene  is 
used  in  manufacturing  films,  coatings,  trash  bags,  and  throwaway  products.  High- 
density  polyethylene  is  used  for  making  injection-molded  parts,  tubes,  sheets,  and 
tanks  that  are  used  for  keeping  chemicals.  The  applications  of  the  ultrahigh  molec- 
ular weight  (UHMW)  polyethylene  include  wear  plates  and  guide  rails  for  filling 
and  packaging  equipment. 

2.  Polypropylene. 


H  H 


H         Chi, 


284 


8      Plastics 


Polypropylene  is  a  thermoplastic  material.  A  molecule  of  this  polymer  has  all 
substituent  groups  (i.e.,  CH3)  on  only  one  of  its  sides.  This  promotes  crystallinity 
and,  therefore,  leads  to  strength  higher  than  that  of  polyethylene.  The  resistance  of 
polypropylene  to  chemicals  is  also  good. 

Polypropylene  is  mainly  used  for  making  consumer  goods  that  are  subjected  to 
loads  during  their  service  life,  such  as  ropes,  bottles,  and  parts  of  appliances.  This 
polymer  is  also  used  in  tanks  and  conduits  because  of  its  superior  resistance  to 
chemicals. 

3.  Polybutylene. 


H         CH3 

C C- 

H         CHo 


Polybutylene  is  a  thermoplastic  polymer  that  has  high  tear,  impact,  and  creep 
resistances.  It  also  possesses  good  wear  properties  and  is  not  affected  by  chemicals. 
Polybutylene  resins  are  available  in  many  grades,  giving  a  wide  range  of  properties 
and,  therefore,  applications. 

The  properties  of  polybutylene  have  made  it  an  appropriate  material  for  piping 
applications.  These  pipes  can  be  joined  together  by  heat  fusion  welding  or  by  me- 
chanical compression.  Some  grades  of  polybutylene  are  used  as  high-performance 
films  for  food  packaging  and  industrial  sheeting. 

4.  Polyvinyl  chloride. 


H  H 


H  CI 


Polyvinyl  chloride  (PVC)  is  a  thermoplastic  polymer  that  can  be  processed  by 
a  variety  of  techniques  like  injection  molding,  extrusion,  blow  molding,  and  com- 
pression molding.  It  is  fairly  weak  and  extremely  notch-sensitive  but  has  excellent 
resistance  to  chemicals.  When  plasticized  (i.e.,  additives  are  used  to  lubricate  the 
molecules),  it  is  capable  of  withstanding  large  strains. 

The  applications  of  rigid  PVC  include  low-cost  piping,  siding,  and  related  pro- 
files, toys,  dinnerware,  and  credit  cards.  Plasticized  PVC  is  used  in  upholstery,  im- 
itation leather  for  seat  covers  and  rainwear,  and  as  insulating  coatings  on  wires. 


8.3  Polymeric  Systems 


285 


5.  Polyvinyleidene  chloride. 


CH2 C  - 

CI 


Polyvinyleidene  chloride  (PVDC)  is  nonpermeable  to  moisture  and  oxygen.  It 
also  possesses  good  creep  properties.  It  is  a  preferred  food-packaging  material 
(e.g.,  saran  wrap).  Rigid  grades  are  used  for  hot  piping. 

6.  Polystyrene. 


CH, 


This  thermoplastic  polymer  is  known  as  "the  cheap  plastic."  It  has  poor  me- 
chanical properties,  can  tolerate  very  little  deflection,  and  breaks  easily.  Because  of 
its  cost,  polystyrene  is  used  for  cheap  toys  and  throwaway  articles.  It  is  also  made 
in  the  form  of  foam  (Styrofoam)  for  sound  attenuation  and  thermal  insulation. 

7.  Polymethyl  methacrylate  (Plexiglas  acrylics). 


CH 
CH2 C 

,C 


O  OCH3 


286 


8      Plastics 


This  polymer  has  reasonably  good  toughness,  good  stiffness,  and  exceptional 
resistance  to  weather.  In  addition,  it  is  very  clear  and  has  a  white-light  transmission 
equal  to  that  of  clear  glass.  Consequently,  this  polymer  finds  application  in  safety 
glazing  and  in  the  manufacture  of  guard  and  safety  glasses.  It  is  also  used  in  mak- 
ing automotive  and  industrial  lighting  lenses.  Some  grades  are  used  as  coatings  and 
lacquers  on  decorative  parts. 

8.  Fluorocarbons  like  polytetrafluoroethylene  (Teflon). 


F  F 

F  F 


Teflon  is  characterized  by  its  very  low  coefficient  of  friction  and  by  the  fact 
that  even  sticky  substances  cannot  adhere  to  it  easily.  It  is  also  the  most  chemi- 
cally inert  polymer.  Nevertheless,  it  has  some  disadvantages,  including  low 
strength  and  poor  processability.  Because  of  its  low  coefficient  of  friction,  Teflon 
is  commonly  used  as  a  dry  film  lubricant.  It  is  also  used  as  lining  for  chemical 
and  food-processing  containers  and  conduits. 

Polycarbonate  group.  These  are  actually  polyesters.  They  are  thermoplastic  and  have 
linear  molecular  chains.  Polycarbonate  exhibits  good  toughness,  good  creep  resistance, 
and  low  moisture  absorption.  It  also  has  good  chemical  resistance.  It  is  widely  used  in 
automotive  and  medical  and  food  packaging  because  of  its  cost  effectiveness.  It  is  also 
considered  to  be  a  high-performance  polymer  and  has  found  application  in  the  form  of 
solar  collectors,  helmets,  and  face  shields. 

Polyacetal  group.     Included  in  this  group  is  formaldehyde,  with  ending  groups. 


HOCH, 


CH,OH 


Formaldehyde  is  a  thermoplastic  polymer  that  can  be  easily  processed  by  injection 
molding  and  extrusion.  It  has  a  tendency  to  be  highly  crystalline,  and,  as  a  result,  this 
polymer  possesses  good  mechanical  properties.  It  also  has  good  wear  properties  and  a 
good  resistance  to  moisture  absorption. 

Its  applications  involve  parts  that  were  made  of  nonferrous  metals  (like  zinc, 
brass,  or  aluminum)  by  casting  or  stamping.  These  applications  are  exemplified  by 
shower  heads,  shower-mixing  valves,  handles,  good-quality  toys,  and  lawn  sprinklers. 


8.3  Polymeric  Systems 


287 


Cellulosic  group.    The  monomer  is  cellulose. 


CH,OH 


H      OH 


Cellulose  itself  is  not  a  thermoplastic  polymer.  It  can  be  produced  by  the  viscous 
regeneration  process  to  take  the  form  of  a  fiber  as  in  rayon,  or  a  thin  film,  as  in  cello- 
phane. Cellophane  applications  involve  mainly  decoration.  Nevertheless,  cellulose  can 
be  chemically  modified  to  produce  the  following  thermoplastics: 

1.  Cellulose  nitrate.  Good  dimensional  stability  and  low  water  absorption  are  the  posi- 
tive characteristics  of  this  polymer.  The  major  disadvantage  that  limits  its  widespread 
use  is  its  inflammability.  Cellulose  nitrate  is  used  in  making  table-tennis  balls,  fash- 
ion accessories,  and  decorative  articles.  It  is  also  used  as  a  base  for  lacquer  paints. 

2.  Cellulose  acetate.  This  polymer  has  good  optical  clarity,  good  dimensional  stabil- 
ity, and  resistance  to  moisture  absorption.  The  uses  of  cellulose  acetate  include 
transparent  sheets  and  films  for  graphic  art,  visual  aids,  and  a  base  for  photographic 
films.  It  is  also  used  in  making  domestic  articles. 

3.  Cellulose  acetate  butyrate.  This  thermoplastic  polymer  is  tough,  has  good  surface 
quality  and  color  stability,  and  can  readily  be  vacuum  formed.  It  finds  popular  use 
in  laminating  with  thin  aluminum  foil. 

4.  Cellulose  acetate  propionate.  This  thermoplastic  polymer  has  reasonably  good 
mechanical  properties  and  can  be  injection  molded  or  vacuum  formed.  It  is  used  for 
blister  packages,  lighting  fixtures,  brush  handles,  and  other  domestic  articles. 

Polyamide  group.    This  family  includes  high-performance  melt-processable  thermo- 
plastics. 


NHR 


R  is  a  chemical  group  that  differs  for  different  members  of  this  family. 

One  group  of  common  polyamides  is  the  nylons.  These  are  characterized  by  their 
endurance  and  retention  of  their  good  mechanical  properties  even  at  relatively  high 
temperatures.  They  also  possess  good  lubricity  and  resistance  to  wear.  The  chief  limi- 
tation is  their  tendency  to  absorb  moisture  and  change  size. 


288 


8     Plastics 


These  polymers  find  use  in  virtually  every  market  (e.g.,  automotive,  electrical, 
wire,  packaging,  and  appliances).  Typical  applications  include  structural  components 
up  to  10  pounds  (4  kg),  bushings,  gears,  cams,  and  the  like. 

ABS.  The  three  monomers  are  acrylonitrile,  butadiene,  and  styrene.  Based  on  this 
three-monomer  system  (similar  to  an  alloy  in  the  case  of  metals),  the  properties  of  this 
group  vary  depending  upon  the  components.  Fifteen  different  types  are  commercially 
used.  They  possess  both  good  mechanical  properties  and  processability.  Applications 
of  the  ABS  group  include  pipes  and  fittings,  appliances  and  automotive  uses,  tele- 
phones, and  components  for  the  electronics  industry. 

Polyesters.  These  polymers  result  from  a  condensation  reaction  of  an  acid  and  an  al- 
cohol. The  type  and  nature  of  the  polymer  obtained  depend  upon  the  acid  and  alcohol 
used.  This  multitude  of  polymers  are  mostly  thermoplastic  and  can  be  injection  molded 
and  formed  into  films  and  fibers.  Their  uses  include  bases  for  coatings  and  paints, 
ropes,  fabrics,  outdoor  applications,  construction,  appliances,  and  electrical  and  elec- 
tronic components.  Polyester  is  also  used  as  a  matrix  resin  for  fiberglass  to  yield  the 
composite  fiber-reinforced  polymer. 

Phenolic  group.    The  monomer  is  phenol  formaldehyde. 


As  previously  mentioned,  phenolics  are  actually  the  oldest  manufactured  ther- 
mosetting polymers.  They  are  processed  by  compression  molding,  where  a  product 
with  a  highly  cross-linked  chain  structure  is  finally  obtained.  Phenolics  are  character- 
ized by  their  high  strength  and  their  ability  to  tolerate  temperatures  far  higher  than 
their  molding  temperature. 

Phenolics  are  recommended  for  use  in  hostile  environments  that  cannot  be  toler- 
ated by  other  polymers.  They  are  used  in  electrical  switchplates,  electrical  boxes,  and 
similar  applications.  Nevertheless,  the  chief  field  of  application  is  as  bonding  agents 
for  laminates,  plywood-grinding  wheels,  and  friction  materials  for  brake  lining. 

Polyimides.    Polyimides  are  mostly  thermosetting  and  have  very  complex  structures. 
They  are  considered  to  be  one  of  the  most  heat-resisting  polymers.  They  do  not  melt 


8.3  Polymeric  Systems  289 

and  flow  at  elevated  temperatures  and  are,  therefore,  manufactured  by  powder  metal- 
lurgy techniques. 

The  polyimides  are  good  substitutes  for  ceramics.  Applications  include  jet-engine 
and  turbine  parts,  gears,  coil  bobbins,  cages  for  ball  bearings,  bushings  and  bearings, 
and  parts  that  require  good  electrical  and  thermal  insulation. 

Epoxies.  Epoxies  and  epoxy  resins  are  a  group  of  polymers  that  become  highly  cross- 
linked  by  reaction  with  curing  agents  or  hardeners.  These  polymers  have  low  molecu- 
lar weight  and  got  their  name  from  the  epoxide  group  at  the  ends  of  the  molecular 
chains.  Epoxy  resins  are  thermosetting  and  have  good  temperature  resistance.  They  ad- 
here very  well  to  a  variety  of  substrates.  Another  beneficial  characteristic  is  their  sta- 
bility of  dimensions  upon  curing. 

The  common  application  of  epoxy  resins  is  as  adhesives.  With  the  addition  of 
fibers  and  reinforcements,  laminates  and  fiber-reinforced  epoxy  resins  can  be  obtained 
and  are  used  for  structural  applications. 

Polyurethanes.  Polyurethanes  involve  a  wide  spectrum  of  polymers  ranging  from 
soft  thermoplastic  elastomers  to  rigid  thermosetting  foams.  While  all  polyurethanes  are 
products  of  a  chemical  reaction  of  an  isocyanate  and  an  alcohol,  different  polymers  are 
apparently  obtained  by  different  reacting  materials. 

Elastomers  are  used  as  die  springs,  forming-die  pads,  and  elastomer-covered  rolls. 
Some  of  these  elastomers  are  castable  at  room  temperature  and  find  popular  applica- 
tion in  rubber  dies  for  the  forming  of  sheet  metals.  Flexible  foam  has  actually  replaced 
latex  rubber  in  home  and  auto  seating  and  interior  padding.  The  rigid  thermosetting 
foam  is  used  as  a  good  insulating  material  and  for  structural  parts.  Other  applications 
of  polyurethanes  include  coating,  varnishes,  and  the  like. 

Silicones.  In  this  group  of  polymers,  silicon  forms  the  backbone  of  the  molecular 
chain  and  plays  the  same  role  as  that  of  carbon  in  other  polymers. 


Silicones  can  be  oils,  elastomers,  thermoplastics,  or  thermosets,  depending  upon 
the  molecular  weight  and  the  functional  group.  Nevertheless,  they  are  all  characterized 
by  their  ability  to  withstand  elevated  temperatures  and  their  water-repellent  properties. 

Silicones  in  all  forms  are  mainly  used  for  high-temperature  applications.  These  in- 
clude binders  for  high-temperature  paints  and  oven  and  good-handling  tubing  gaskets. 
Silicone  oils  are  used  as  high-temperature  lubricants,  mold  release  agents,  and  damp- 
ing or  dielectric  fluids. 

Elastomers.  These  polymeric  materials  possess  a  percentage  elongation  of  greater 
than  100  percent  together  with  significantly  high  resilience.  This  rubberlike  behavior 
is  attributed  to  the  branching  of  the  molecular  chains.  Elastomers  mainly  include  five 


290  8      Plastics 


polymers:  natural  rubber,  neoprene,  silicone  rubber,  polyurethane,  and  fiuoroelas- 
tomers.  Natural  rubber  is  extracted  as  thick,  milky  liquid  from  a  tropical  tree.  Next, 
moisture  is  removed,  additives  (coloring,  curing  agents,  and  fillers)  are  blended  with  it, 
and  the  mixture  is  then  vulcanized.  The  latter  operation  involves  heating  up  to  a  tem- 
perature of  300°F  (150°C)  to  start  cross-linking  and  branching  reactions. 

The  application  of  elastomers  includes  seals,  gaskets,  oil  rings,  and  parts  that  pos- 
sess rubberlike  behavior  such  as  tires,  automotive  and  aircraft  parts,  and  parts  in  form- 
ing dies. 

Additives 

Additives  are  materials  that  are  compounded  with  polymers  in  order  to  impart  and/or 
enhance  certain  physical,  chemical,  manufacturing,  or  mechanical  properties.  They  are 
also  sometimes  added  just  for  the  sake  of  reducing  the  cost  of  products.  Commonly 
used  additives  include  fillers,  plasticizers,  lubricants,  colorants,  antioxidants,  and  sta- 
bilizers. 

Fillers.  Fillers  involve  wood  flour,  talc,  calcium  carbonate,  silica,  mica  flour,  cloth, 
and  short  fibers  of  glass  or  asbestos.  Fillers  have  recently  gained  widespread  industrial 
use  as  a  result  of  the  continued  price  increase  and  short  supply  of  resin  stocks.  An  ex- 
pensive or  unavailable  polymer  can  sometimes  be  substituted  by  another  filled  poly- 
mer, provided  that  an  appropriate  filler  material  is  chosen. 

The  addition  of  inorganic  fillers  usually  tends  to  increase  the  strength  because  this 
kind  of  additive  inhibits  the  mobility  of  the  polymers'  molecular  chains.  Nevertheless, 
if  too  much  filler  is  added,  it  may  create  enclaves  or  weak  spots  and  cause  problems 
during  processing,  especially  if  injection  molding  is  employed. 

Plasticizers.  Plasticizers  are  organic  chemicals  (high-boiling-temperature  solvents) 
that  are  admixed  with  polymers  in  order  to  enhance  resilience  and  flexibility.  This  is  a  re- 
sult of  facilitating  the  mobility  of  the  molecular  chains,  thus  enabling  them  to  move  eas- 
ily relative  to  one  another.  On  the  other  hand,  plasticizers  reduce  the  strength.  Therefore, 
a  polymer  that  meets  requirements  without  the  addition  of  plasticizers  is  the  one  to  use. 

Lubricants.  Lubricants  are  chemical  substances  that  are  added  in  small  quantities  to 
the  polymer  to  improve  processing  and  flowability.  They  include  fatty  acids,  fatty  al- 
cohols, fatty  esters,  metallic  stearates,  paraffin  wax,  and  silicones.  Lubricants  are  clas- 
sified as  external  (applied  externally  to  the  polymer),  internal,  or  internal-external.  The 
last  group  includes  most  commercially  used  lubricants. 

Colorants.  Colorants  may  be  either  dyes  or  pigments.  Dyes  have  smaller  molecules 
and  are  transparent  when  dissolved.  Pigment  particles  are  relatively  large  (over  1  |im) 
and  are,  therefore,  either  translucent  or  opaque.  Pigments  are  more  widely  used  than 
dyes  because  dyes  tend  to  extrude  from  the  polymers. 

Antioxidants.  The  use  of  antioxidants  is  aimed  at  enhancing  the  resistance  to  oxida- 
tion and  degradation  of  polymers,  thus  extending  their  useful  temperature  range  and 
service  life.  These  substances  retard  the  chemical  reactions  that  are  caused  by  the  pres- 
ence of  oxygen. 


8.4  Processing  of  Plastics  291 

Stabilizers.  Stabilizers  are  substances  that  are  added  to  polymers  to  prevent  degrada- 
tion as  a  result  of  heat  or  ultraviolet  rays.  The  mechanism  of  inhibiting  degradation  of 
polymers  differs  for  different  stabilizers.  However,  ultraviolet  stabilizers  usually  func- 
tion by  absorbing  ultraviolet  radiation. 


8.4  PROCESSING  OF  PLASTICS 


A  variety  of  processing  methods  can  be  employed  in  manufacturing  plastic  products. 
However,  it  must  be  kept  in  mind  that  no  single  processing  method  can  successfully  be 
employed  in  shaping  all  kinds  of  plastics.  Each  process  has  its  own  set  of  advantages 
and  disadvantages  that  influence  product  design.  Following  is  a  survey  of  the  common 
methods  for  plastic  processing. 

Casting 

Casting  is  a  fairly  simple  process  that  requires  no  external  force  or  pressure.  It  is  usually 
performed  at  room  temperature  and  involves  filling  the  mold  cavity  with  monomers  or 
partially  polymerized  syrups  and  then  heating  to  cure.  After  amorphous  solidification, 
the  material  becomes  isotropic,  with  uniform  properties  in  all  directions.  Nevertheless, 
a  high  degree  of  shrinkage  is  experienced  during  solidification  and  must  be  taken  into 
consideration  when  designing  the  mold.  Sheets,  rods,  and  tubes  can  be  manufactured  by 
casting,  although  the  typical  application  is  in  trial  jigs  and  fixtures  as  well  as  in  insulat- 
ing electrical  components.  Acrylics,  epoxies,  polyesters,  polypropylene,  nylon,  and 
PVC  can  be  processed  by  casting.  The  casting  method  employed  is  sometimes  modified 
to  suit  the  kind  of  polymer  to  be  processed.  Whereas  nylon  is  cast  in  its  hot  state  after 
adding  a  suitable  catalyst,  PVC  film  is  produced  by  solution  casting.  This  process  in- 
volves dissolving  the  PVC  into  an  appropriate  solvent,  pouring  the  solution  on  a  sub- 
strate, and  allowing  the  solvent  to  evaporate  in  order  to  finally  obtain  the  required  film. 

Blow  Molding 

Blow  molding  is  a  fast,  efficient  method  for  producing  hollow  containers  of  thermo- 
plastic polymers.  The  hollow  products  manufactured  by  this  method  usually  have  thin 
walls  and  range  in  shape  and  size  from  small,  fancy  bottles  to  automobile  fuel  tanks. 
Although  there  are  different  versions  of  the  blow  molding  process,  they  basically 
involve  blowing  a  tubular  shape  (parison)  of  heated  polymer  in  a  cavity  of  a  split  mold. 
As  can  be  seen  in  Figure  8.5,  air  is  injected  through  a  needle  into  the  parison,  which 
expands  in  a  fairly  uniform  thickness  and  finally  conforms  to  the  shape  of  the  cavity. 

Injection  Molding 

Injection  molding  is  the  most  commonly  used  method  for  mass  production  of  plastic 
articles  because  of  its  high  production  rates  and  the  good  control  over  the  dimensions 
of  the  products.  The  process  is  used  for  producing  thermoplastic  articles,  but  it  can  also 
be  applied  to  thermosets.  The  main  limitation  of  injection  molding  is  the  required  high 


292 


8      Plastics 


FIGURE    8.5 

The  blow  molding 
process 


M 


/ 

Separation 
line 


View  normal  to 
the  separation  line 


initial  capital  cost,  which  is  due  to  the  expensive  machines  and  molds  employed  in  the 
process. 

The  process  basically  involves  heating  the  polymer,  which  is  fed  from  a  hopper  in 
granular  pellet  or  powdered  forms,  to  a  viscous  melted  state  and  then  forcing  it  into  a 
split-mold  cavity,  where  it  hardens  under  pressure.  Next,  the  mold  is  opened,  and  the 
product  is  ejected  by  a  special  mechanism.  Molds  are  usually  made  of  tool  steel  and 
may  have  more  than  a  single  cavity. 

Figure  8.6  shows  a  modern  screw-preplasticator  injection  unit  employed  in  injection 
molding  of  thermoplastics.  As  can  be  seen,  the  diverter  valve  allows  the  viscous  polymer 
to  flow  either  from  the  plasticating  screw  to  the  pressure  cylinder  or  from  the  cylinder  to 
the  cooled  mold.  When  thermosets  are  to  be  injection  molded,  a  machine  with  a  differ- 
ent design  has  to  be  used.  Also,  the  molds  must  be  hot  so  that  the  polymer  can  cure. 

Once  the  decision  has  been  made  to  manufacture  a  plastic  product  by  injection 
molding,  the  product  designer  should  make  a  design  that  facilitates  and  favors  this 
process.  Following  are  some  design  considerations  and  guidelines. 

Make  the  thickness  of  a  product  uniform  and  as  small  as  possible.  Injection  mold- 
ing of  thermoplastics  produces  net-shaped  parts  by  going  from  a  liquid  state  to  a  solid 
state.  (These  net-shaped  parts  are  used  as  manufactured;  they  do  not  require  further 
processing  or  machining.)  This  requires  time  to  allow  the  heat  to  dissipate  so  that  the 


FIGURE    8.6 

The  injection  molding 
process 


Heaters 


Split 
mold 


njection 
plunger 


8.4  Processing  of  Plastics 


293 


polymer  melt  can  solidify.  The  thicker  the  walls  of  a  product,  the  longer  the  product 
cycle,  and  the  higher  its  cost.  Consequently,  a  designer  has  to  keep  the  thickness  of  a 
product  to  a  minimum  without  jeopardizing  the  strength  and  stiffness  considerations. 
Also,  thickness  must  always  be  kept  uniform;  if  change  in  thickness  is  unavoidable,  it 
should  be  made  gradually.  It  is  better  to  use  ribs  rather  than  increase  the  wall  thickness 
of  a  product.  Figure  8.7  shows  examples  of  poor  design  and  how  they  can  be  modified 
(by  slight  changes  in  constructional  details)  so  that  sound  parts  are  produced. 

Provide  generous  fillet  radii.  Plastics  are  generally  notch-sensitive.  The  designer 
should,  therefore,  avoid  sharp  corners  for  fillets  and  provide  generous  radii  instead. 
The  ratio  of  the  fillet  radius  to  the  thickness  should  be  at  least  1 .4. 

Ensure  that  holes  will  not  require  complex  tooling.  Holes  are  produced  by  using  core 
pins.  It  is,  therefore,  clear  that  through  holes  are  easier  to  make  than  blind  holes.  Also, 
when  blind  holes  are  normal  to  the  flow,  they  require  retractable  core  pins  or  split 
tools,  thus  increasing  the  production  cost. 


FIGURE    8.7 

Examples  of  poor  and 
good  designs  of  walls 
of  plastic  products 


V&77A      WZZ& 


Yes 


Better 


Rib 


Improved  design 


V////////////A 


Poor 


1.5/ 


Good 


Better 


294 


8      Plastics 


FIGURE    8.8 

Examples  of  poor  and 
good  designs  of  bosses 
in  injection-molded 
parts 


7. 


^y\ 


77777 


Poor  design 


Good  design 


a 


D 


Through  holes  are  better  than  blind  holes 


Provide  appropriate  draft.  As  is  the  case  with  forging,  it  is  important  to  provide  a 
draft  of  1  °  so  that  the  product  can  be  injected  from  the  mold. 

Avoid  heavy  sections  when  designing  bosses.  Heavy  sections  around  bosses  lead  to 
wrappage  and  dimensional  control  problems.  Figure  8.8  shows  poor  and  good  designs 
of  bosses. 


Compression  Molding 

Compression  molding  is  used  mainly  for  processing  thermosetting  polymers.  The 
process  involves  enclosing  a  premeasured  charge  of  polymer  within  a  closed  mold  and 
then  subjecting  that  charge  to  combined  heat  and  pressure  until  it  takes  the  shape  of  the 
mold  cavity  and  cures.  Figure  8.9  shows  a  part  being  produced  by  this  process. 

Although  the  cycle  time  for  compression  molding  is  very  long  when  compared 
with  that  for  injection  molding,  the  process  has  several  advantages.  These  include  low 
capital  cost  (because  the  tooling  and  the  equipment  used  are  simpler  and  cheaper)  and 
the  elimination  of  the  need  for  sprues  or  runners,  thus  reducing  the  material  waste.  There 


8.4  Processing  of  Plastics 


295 


FIGURE    8.9 

The  compression  moldin 
process 


Flash 


Part 


are,  however,  limitations  upon  the  shape  and  size  of  the  products  manufactured  by  this 
method.  It  is  generally  difficult  to  produce  complex  shapes  or  large  parts  as  a  result  of 
the  poor  flowability  and  long  curing  times  of  the  thermosetting  polymers. 

Transfer  Molding 

Transfer  molding  is  a  modified  version  of  the  compression  molding  process,  and  it  is 
aimed  at  increasing  the  productivity  by  accelerating  the  production  rate.  As  can  be 
seen  in  Figure  8.10,  the  process  involves  placing  the  charge  in  an  open,  separate  "pot," 
where  the  thermosetting  polymer  is  heated  and  forced  through  sprues  and  runners  to 
fill  several  closed  cavities.  The  surfaces  of  the  sprues,  runners,  and  cavities  are  kept  at 
a  temperature  of  280  to  300°F  (140  to  200°C)  to  promote  curing  of  the  polymer.  Next, 
the  entire  shot  (i.e.,  sprues,  runners,  product,  and  the  excess  polymer  in  the  pot)  is 
ejected. 

Rotational  Molding 

Rotational  molding  is  a  process  by  which  hollow  objects  can  be  manufactured  from 
thermoplastics  and  sometimes  thermosets.  It  is  based  upon  placing  a  charge  of  solid  or 
liquid  polymer  in  a  mold.  The  mold  is  heated  while  being  rotated  simultaneously 
around  two  perpendicular  axes.  As  a  result,  the  centrifugal  force  pushes  the  polymer 
against  the  walls  of  the  mold,  thus  forming  a  homogeneous  layer  of  uniform  thickness 


FIGURE    8.10 

The  transfer  molding 
process 


Plunger 


Part 


Runner 


296 


8      Plastics 


FIGURE    8.11 

The  extrusion  process 


Changeable  die 


Extruded  section 

4 


Heater 


that  conforms  to  the  shape  of  the  mold,  which  is  then  cooled  before  the  product  is 
ejected.  The  process,  which  has  a  relatively  long  cycle  time,  has  the  advantage  of  of- 
fering almost  unlimited  product  design  freedom.  Complex  parts  can  be  molded  by  em- 
ploying low-cost  machinery  and  tooling. 

Extrusion 

In  extrusion,  a  thermoplastic  polymer  in  powdered  or  granular  form  is  fed  from  a  hop- 
per into  a  heated  barrel,  where  the  polymer  melts  and  is  then  extruded  out  of  a  die.  Fig- 
ure 8.11  shows  that  plastics  extrusion  is  a  continuous  process  capable  of  forming  an 
endless  product  that  has  to  be  cooled  by  spraying  water  and  then  cut  to  the  desired 
lengths.  The  process  is  employed  to  produce  a  wide  variety  of  structural  shapes,  such 
as  profiles,  channels,  sheets,  pipes,  bars,  angles,  films,  and  fibers.  Extrusions  like  bars, 
sheets,  and  pipes  can  also  be  further  processed  by  other  plastic  manufacturing  methods 
until  the  desired  final  product  is  obtained. 

A  modification  of  conventional  extrusion  is  a  process  known  as  coextrusion.  It  in- 
volves extruding  two  or  more  different  polymers  simultaneously  in  such  a  manner  that 
one  polymer  flows  over  and  adheres  to  the  other  polymer.  This  process  is  used  in  in- 
dustry to  obtain  combinations  of  polymers,  each  contributing  some  desired  property. 
Examples  of  coextrusion  include  refrigerator  liners,  foamed-core  solid-sheath  tele- 
phone wires,  and  profiles  involving  both  dense  material  and  foam,  which  are  usually 
used  as  gasketing  in  automotive  and  appliance  applications. 


Thermoforming 

Thermoforming  involves  a  variety  of  processes  that  are  employed  to  manufacture  cup- 
like products  from  thermoplastic  sheets  by  a  sequence  of  heating,  forming,  cooling, 
and  trimming.  First,  the  sheet  is  clamped  all  around  and  heated  to  the  appropriate  tem- 
perature by  electric  heaters  located  above  it.  Next,  the  sheet  is  stretched  under  the  ac- 
tion of  pressure,  vacuum,  or  male  tooling  and  is  forced  to  take  the  shape  of  a  mold. 
The  polymer  is  then  cooled  to  retain  the  shape.  This  is  followed  by  removing  the  part 
from  the  mold  and  trimming  the  web  surrounding  it.  Figure  8.12a  through  d  illustrates 
the  different  thermoforming  processes. 

Although  thermoforming  was  originally  developed  for  the  low-volume  production 
of  containers,  the  process  can  be  automated  and  made  suitable  for  high-volume  appli- 
cations. In  this  case,  molds  are  usually  made  of  aluminum  because  of  its  high  thermal 


8.4  Processing  of  Plastics 


297 


FIGURE    8.12 

Different 
thermoforming 
processes:  (a)  straight 
vacuum  forming; 

(b)  drape  forming; 

(c)  matched-mold 
forming;  (d)  vacuum 
snapback 


Heater 


Original 
sheet 


Heater 


-■•»fi"»-»  in »»-' 

v  ^---CLOngina 

sheet 
Mold 


(Upper  half 
of  mold) 


Lower  half 
of  mold) 


atmosphere  ~| 


Vent  for 

relieving 

entrapped  air 


(0 


Hi 


Heated 
plastic 
sheet 


Vacuum 


[1)   First  stage 


Vacuum 


u 


¥fm 


MmtM 


Atmosphere 


(2)   Second  stage 
(d) 


conductivity.  For  low-volume  or  trial  production,  molds  are  made  of  wood  or  even 
plaster  of  paris. 

Examples  of  the  parts  produced  by  thermoforming  include  containers,  panels, 
housings,  machine  guards,  and  the  like.  The  only  limitation  on  the  shape  of  the  prod- 
uct is  that  it  should  not  contain  holes.  If  holes  are  absolutely  required,  they  should  be 
made  by  machining  at  a  later  stage. 

Calendering 

Calendering  is  the  process  employed  in  manufacturing  thermoplastic  sheets  and  films. 
This  process  is  similar  to  rolling  with  a  four-high  rolling  mill,  except  that  the  rolls  that 
squeeze  the  polymer  are  heated.  The  thermoplastic  sheet  is  fed  and  metered  in  the  first 
and  second  roll  gaps,  whereas  the  third  roll  gap  is  devoted  to  gaging  and  finishing. 


298  8      Plastics 


FIGURE    8.13 

The  calendering 
process 


Most  of  the  calendering  products  are  flexible  or  rubberlike  sheets  and  films,  although 
the  process  is  sometimes  applied  to  ABS  and  polyethylene.  Figure  8.13  illustrates  the 
calendering  process. 

Machining  of  Plastics 

In  some  cases,  thermoplastic  and  thermosetting  polymers  are  subjected  to  machining 
operations  like  sawing,  drilling,  or  turning.  Some  configurations  and  small  lot  sizes  can 
be  more  economically  achieved  by  machining  than  by  any  other  plastic-molding 
method.  Nevertheless,  there  are  several  problems  associated  with  the  machining  of  plas- 
tics. For  instance,  each  type  of  plastic  has  its  own  unique  machining  characteristics,  and 
they  are  very  different  from  those  of  the  conventional  metallic  materials.  A  further  prob- 
lem is  the  excessive  tool  wear  experienced  when  machining  plastics,  which  results  in  the 
interruption  of  production  as  well  as  additional  tooling  cost.  Although  much  research  is 
needed  to  provide  solutions  for  these  problems,  there  are  some  general  guidelines: 

1.  Reduce  friction  at  the  tool-workpiece  interface  by  using  tools  with  honed  or  pol- 
ished surfaces. 

2.  Select  tool  geometry  so  as  to  generate  continuous-type  chips.  Recent  research  has 
revealed  that  there  exists  a  critical  rake  angle  (see  Chapter  9)  that  depends  upon 
the  polymer,  depth  of  cut,  and  cutting  speed. 

3.  Use  twist  drills  that  have  wide,  polished  flutes,  low  helix  angles,  and  tool-point 
angles  of  about  70°  and  120°. 

Recently,  lasers  have  been  employed  in  cutting  plastics.  Because  a  laser  acts  as  a  ma- 
terials eliminator,  its  logical  application  is  cutting  and  hole  drilling.  High-pressure  water 
jets  also  currently  find  some  application  in  the  cutting  of  polymers  and  composites. 

Welding  of  Plastics 

There  are  several  ways  for  assembling  plastic  components.  The  commonly  used  meth- 
ods include  mechanical  fastening,  adhesive  bonding,  thermal  bonding,  and  ultrasonic 
welding.  Only  thermal  bonding  and  ultrasonic  welding  are  discussed  next  because  the 
first  two  operations  are  similar  to  those  used  with  metals. 


8.4  Processing  of  Plastics 


299 


FIGURE    8.14 

Steps  involved  in  hot- 
plate joining 


Fixtures 


Hot 
plate 


Force 


Thermal  bonding  of  plastics.  Thermal  bonding,  which  is  also  known  as  fusion  bond- 
ing, involves  the  melting  of  the  weld  spots  in  the  two  plastic  parts  to  be  joined  and  then 
pressing  them  together  to  form  a  strong  joint.  Figure  8.14  illustrates  the  steps  involved 
in  the  widely  used  thermal  bonding  method  known  as  hot-plate  joining.  As  can  be  seen 
in  the  figure,  a  hot  plate  is  inserted  between  the  edges  to  be  mated  in  order  to  melt  the 
plastic  parts;  melting  stops  when  the  plate  comes  in  contact  with  the  holding  fixture. 
Next,  the  plate  is  withdrawn,  and  the  parts  are  pressed  together  and  left  to  cool  to  yield 
a  strong  joint.  The  cycle  time  usually  ranges  from  15  to  20  seconds,  depending  upon 
the  relationship  between  the  melt  time  and  the  temperature  (of  the  hot  plate)  for  the 
type  of  plastic  to  be  bonded.  Also,  this  process  is  applied  only  to  thermoplastics. 

Figure  8.15  illustrates  different  types  of  joint  design.  The  one  to  select  is  depen- 
dent upon  both  the  desired  strength  and  the  appearance  of  the  joint.  The  product  de- 
signer must  keep  in  mind  that  a  small  amount  of  material  is  displaced  from  each  side 
to  form  the  weld  bead.  This  must  be  taken  into  account  when  dimensional  tolerance  is 
critical,  such  as  when  fusion-bonded  parts  are  to  be  assembled  together. 

Another  thermal  bonding  process,  which  is  equivalent  to  riveting  in  the  case  of 
metals,  is  referred  to  as  the rmo staking.  As  can  be  seen  in  Figure  8.16,  the  process 


FIGURE    8.15 

Different  joint  designs 
for  fusion  bonding 


1 


Straight 
butt  joint 


Flanged 
butt  joint 


Bead  enclosed 


Bead  covered 


Recessed  weld 


300 


8      Plastics 


FIGURE    8.16 

The  thermostaking 
process 


Hot  air 


involves  the  softening  of  a  plastic  stud  by  a  stream  of  hot  air  and  then  forming  the 
softened  stud  and  holding  it  while  it  cools  down.  Thermal  bonding  processes  find 
widespread  application  in  the  automotive,  appliance,  battery,  and  medical  industries. 

Ultrasonic  welding  of  plastics.  Ultrasonic  welding  is  gaining  popularity  in  industry 
because  of  its  low  cycle  time  of  about  0.5  second  and  the  strong,  tight  joints  that  are 
easily  obtainable.  The  process  is  used  for  thermoplastics  and  involves  conversion  of 
high-frequency  electrical  energy  to  high-frequency  mechanical  vibrations  that  are,  in 
turn,  employed  to  generate  highly  localized  frictional  heating  at  the  interface  of  the 
mating  parts.  This  frictional  heat  melts  the  thermoplastic  polymer,  allowing  the  two 
surfaces  to  be  joined  together. 

The  product  designer  must  bear  in  mind  that  not  all  thermoplastics  render  them- 
selves suitable  for  ultrasonic  welding.  Whereas  amorphous  thermoplastics  are  good 
candidates,  crystalline  polymers  are  not  suitable  for  this  process  because  they  tend  to 
attenuate  the  vibrations.  Hydroscopic  plastics  (humidity-absorbing  polymers,  such  as 
nylons)  can  also  create  problems  and  must,  therefore,  be  dried  before  they  are  ultra- 
sonically  welded.  In  addition,  the  presence  of  external  release  agents  or  lubricants  re- 
duces the  coefficient  of  friction,  thus  making  ultrasonic  welding  more  difficult. 

The  equipment  used  involves  a  power  supply,  a  transducer,  and  a  horn.  The  power 
supply  converts  the  conventional  115-V,  60-Hz  (or  220- V,  50-Hz)  current  into  a  high- 
frequency  current  (20,000  Hz).  The  transducer  is  usually  a  piezoelectric  device  that 
converts  the  electrical  energy  into  high-frequency,  axial-mechanical  vibrations.  The 
horn  is  the  part  of  the  system  that  is  responsible  for  amplifying  and  transmitting  the 
mechanical  vibrations  to  the  plastic  workpiece.  Horns  may  be  made  of  aluminum, 
alloy  steel,  or  titanium.  The  latter  material  possesses  superior  mechanical  properties 
and  is,  therefore,  used  with  heavy-duty  systems.  The  horns  amplify  the  mechanical  vi- 
bration via  a  continuous  decrease  in  the  cross-sectional  area  and  may  take  different 
forms  to  achieve  that  goal,  as  shown  in  Figure  8.17. 

The  task  of  joint  design  for  ultrasonic  welding  is  critical  because  it  affects  the  de- 
sign of  the  molded  parts  to  be  welded.  Fortunately,  there  are  a  variety  of  joint  designs, 
and  each  has  its  specific  features,  advantages,  and  limitations.  The  type  of  joint  to  be 
used  should  obviously  depend  upon  the  kind  of  plastic,  the  part  geometry,  the  strength 
required,  and  the  desired  cosmetic  appearance.  Following  is  a  discussion  of  the  com- 
monly used  joint  designs,  which  are  illustrated  in  Figure  8.18. 


8.4  Processing  of  Plastics 


301 


FIGURE    8.17 

Different  horn  shapes 
employed  in  ultrasonic 
welding  of  plastics 


\  r 


Catenoidal  horn 


Step  horn 


Exponential  horn 


FIGURE    8.18 

Different  joint  designs 
for  ultrasonic  welding: 
(a)  butt  joint;  (b)  step 
joint;  (c)  tongue-and- 
groove  joint;  (d) 
interference  joint; 
(e)  scarf  joint 


(a) 


(b) 


M 


u 


Parts  to  be 
joined 


Fixture 


(0 


(d) 


s: 


(e) 


1.  Butt  joint  with  energy  director.  The  butt  joint  (see  Figure  8.18a)  is  the  most 
commonly  used  joint  design  in  ultrasonic  welding.  As  can  be  seen  in  the  figure, 
one  of  the  mating  parts  has  a  triangular-shaped  projection.  This  projection  is 
known  as  an  energy  director  because  it  helps  to  limit  the  initial  contact  to  a  very 
small  area,  thus  increasing  the  intensity  of  energy  at  that  spot.  This  causes  the 
projection  to  melt  and  flow  and  cover  the  whole  area  of  the  joint.  This  type  of 
joint  is  considered  to  be  the  easiest  to  produce  because  it  is  not  difficult  to  mold 
into  a  part. 

2.  Step  joint  with  energy  director.  The  step  joint  (see  Figure  8.18b)  is  stronger  than 
the  butt  joint  and  is  recommended  when  cosmetic  appearance  is  desired. 


302 


8      Plastics 


FIGURE    8.19 

Ultrasonic  installation 
of  metal  insert  into 
plastic  part 


Metal  insert 

(diameter  bigger 

than  the  hole) 


Plastic  part 


Fixture 


3.  Tongue-and-groove  joint  with  energy  director.  The  tongue-and-groove  joint  (see 
Figure  8.18c)  promotes  the  self-locating  of  parts  and  prevents  flash.  It  is  stronger 
than  both  of  the  previously  mentioned  methods. 

4.  Interference  joint.  The  interference  joint  (see  Figure  8.18d)  is  a  high-strength  joint 
and  is  usually  recommended  for  square  corners  or  rectangular-shaped  parts. 


FIGURE    8.20 

Ultrasonic  staking 


Staking 
tool 


Flared  stake 
diameter  less  than  ^  in.  (1.6  mm) 


Spherical  stake 
diameter  less  than -^  in.  (1.6  mm) 


^ 


4S 


®m 


Hollow  stake 
diameter  more  than  ^-  in.  (4  mm) 


N/S/VV 


Knurled  stake 

(used  for  high-volume  production 

and/or  where  appearance  and 

strength  are  not  critical) 


Flush  stake 

(recommended  when  the  thickness  of 

the  sheet  allows  a  chamber  or  a  counterbase) 


8.5  Fiber-Reinforced  Polymeric  Composites  303 

5.  Scarf  joint.  The  scarf  joint  (see  Figure  8.18e)  is  another  high-strength  joint  and  is 
recommended  for  components  with  circular  or  oval  shapes. 

In  addition  to  welding,  ultrasonics  are  employed  in  inserting  metallic  parts  into 
thermoplastic  components.  Figure  8.19  illustrates  an  arrangement  for  the  ultrasonic  in- 
stallation of  a  metal  insert  into  a  plastic  part. 

Another  useful  application  of  these  systems  is  ultrasonic  staking,  which  is  equiv- 
alent to  riveting  or  heading.  Figure  8.20  indicates  the  different  types  of  stakes,  as  well 
as  their  recommended  applications.  Notice  that  these  stakes  can  be  flared,  spherical, 
hollow,  knurled,  or  flush. 


J. 5  FIBER-REINFOR 
COMPOSITES 


In  this  present  age  of  new  materials,  at  the  forefront  of  advancing  developments  are 
materials  based  on  the  combination  of  organic  polymer  resins  and  high-strength,  high- 
stiffness  synthetic  fibers.  This  section  addresses  the  materials,  processing,  and  design 
methodology  of  fiber-reinforced  polymeric  composites. 

Historical  Background 

Although  the  merits  of  fiber-reinforced  materials  have  been  known  for  centuries, 
(straw-reinforced  clay  was  reportedly  used  as  a  building  material  by  the  Egyptians  in 
600  B.C.),  it  is  only  in  the  past  40  years  that  fiber-reinforced  polymers  have  become  im- 
portant engineering  materials.  New  synthetic  high-strength,  high-modulus  fibers  and 
new  resins  and  matrix  materials  have  elevated  fiber-reinforced  composites  into  the  ma- 
terial of  choice  for  innovative  lightweight,  high-strength  engineered  products.  These  de- 
velopments along  with  established  engineering  design  criteria  and  special  processing 
technology  have  advanced  fiber-reinforced  composites  close  to  the  realm  of  a  commod- 
ity material  of  construction.  In  the  areas  of  automobile  bodies,  recreational  boat  hulls, 
and  bathroom  fixtures  (bathtubs  and  shower  stalls),  fiberglass-reinforced  organic  poly- 
mer resins  have  indeed  become  the  material  of  choice.  In  more  advanced  applications, 
the  first  completely  fiber-reinforced  polymeric  resin  composite  aircraft  came  into  exis- 
tence in  the  1980s.  For  the  1990s,  some  important  nonaerospace  applications  are  emerg- 
ing, such  as  sports  equipment  (sailboat  spars)  and,  more  recently,  wind  turbine  blades. 
The  utilization  of  composite  materials  in  functional  engineering  applications  con- 
tinues to  grow.  It  is,  therefore,  important  for  engineering  students  to  know  about  and 
understand  these  materials  so  that  new  uses  may  be  developed  and  propagated.  Con- 
sequently, a  brief  review  of  organic  polymer  engineering  composites  is  presented  next. 
A  general  description  of  these  materials,  their  unique  properties,  processing  tech- 
niques, and  engineering  design  features  will  put  into  perspective  present  and  future 
uses  of  fiber-reinforced  polymer  (FRP)  engineering  materials. 


*  Section  8.5  was  written  by  Dr.  Armand  F.  Lewis,  Lecturer  at  the  University  of  Massachusetts  Dartmouth. 


304 


8      Plastics 


Nature  of  Composites 

A  composite  may  be  defined  as  a  material  made  up  of  several  identifiable  phases,  com- 
bined in  an  ordered  fashion  to  provide  specific  properties  different  from  or  superior  to 
those  of  the  individual  materials.  Many  types  of  composites  exist,  including  laminated 
materials,  filamentary-wound  or  -layered  and  particulate-filled  compositions,  and  mul- 
tiphase alloys  and  ceramics.  Most  naturally  occurring  structural  materials  are  compos- 
ites (wood,  stone,  bone,  and  tendon). 

Overall,  composite  materials  can  be  classified  according  to  Table  8.1.  We  will 
focus  on  fiber/resin  composite  materials  composed  of  higher-strength,  higher-modulus 
fibers  embedded  in  an  organic  polymer/resin  matrix.  Table  8.2  lists  some  of  the  com- 
mon resin  and  fiber  materials  employed.  These  composite  materials  are  generally  re- 
ferred to  as  fiber-reinforced  polymers  (FRP).  Currently,  polyester  and  epoxy  resins 
are  the  most  common  commercially  used  matrix  resin  polymers,  while  glass  fibers  are 
the  most  widely  used  reinforcing  fiber.  Resin  matrix  composites  containing  high- 
strength,  high-elasticity-modulus  carbon  (graphite),  polyaramid  (Kevlar,  a  DuPont 
trade  name),  and  boron  fibers  are  also  in  use  for  specialty  (advanced)  composite  mate- 
rial applications. 

The  integral  combination  of  high-strength,  high-elasticity-modulus  fibers  and  rel- 
atively low-strength,  low-rigidity  polymer  matrices  forms  some  unique  engineering 
materials.  FRP  composites  possess  the  material  processing  and  fabrication  properties 
of  polymeric  materials  yet,  due  to  their  fiber  reinforcement,  can  be  designed  to  possess 
directional  stiffness  and  strength  properties  comparable  to  those  of  metals.  These  me- 
chanical properties  can  be  achieved  at  a  very  light  weight.  This  feature  can  be  illus- 
trated by  comparing  the  specific  strength  (tensile  strength/density)  to  the  specific 
elastic  modulus  (tensile  elasticity-modulus/density)  of  various  fiber-reinforced  com- 
posite materials  with  plastics  and  metals.  Figure  8.21  compares  the  specific  strengths 
and  specific  elastic  moduli  of  these  materials.  Notice  that  commodity  elastomers,  plas- 


TABLE    8.1 

Classifications  of 
composite  materials 


Classification 

Typical  Example(s) 

Fiber/resin  composites 

Glass  fabric/mat  reinforced  polyester 

Continuous 

resin  molded  into  sport  boat  hulls 

Discontinuous 

Heterophase  polymer  mixtures 

Aluminum  and/or  graphite  powder 

Random  particulate  filled 

blended  into  nylon  plastic  to  form  a 

Flake  or  shaped  particles 

machine  gear 

Interstitial  polymeric  materials 

"Marbleized"  decorative  plastic  for 

Interpenetrating  polymer  networks 

wall  panels 

Skeletal  composites 

Laminar  and  linear  composites 

High-pressure  laminates  used  in 

Material  hybrids 

kitchen  countertops  and 

Polymer-polymer 

polyurethane  rubber-impregnated 

polyaramid  rope/cable 

8.5  Fiber-Reinforced  Polymeric  Composites 


305 


TABLE    8.2 

Some  materials  used  in 
organic  polymer 
engineering  composites 


/                  Matrix  Resin 

Maximum  Service  Temperature  \ 

Epoxy 

Up  to  121°C  (250°F) 

Polyester 

Up  to  62°C  (non  HT) 

Vinyl  ester 

Up  to  145°C  (HT  type) 

Phenolic 

Up  to  149°C  (300°F) 

Polyimide 

Up  to  260°C  (500°F) 

Thermoplastics 

Nylon 

Up  to  80°C  (175°F) 

Polyphenylene  sulfide 

Up  to  149°C  (300°F) 

Polyetheretherktone  (PEEK) 

Up  to  200°C  (392°F) 

Fiber  (Continuous  Yarn/Filament, 

Woven  Fabric, 

Nonwoven  Mat,  Chopped  Fiber) 

Glass  (especially  E  and  S  glass) 

Polyaramid  organic  fiber  (Kevlar)® 

(Dupont  trademark) 

Carbon 

Boron 

Form  for  Processing 

Liquid  casting  resin 

B-stage  resin  mixture 

Preimpregnated  (prepreg)  B-stage  resin/fiber/fabr 

ic  combination 

tics,  and  metals  occupy  only  a  very  small  portion  of  this  structural  materials  map. 
Fibers  and  fiber-reinforced  resin  composites  occupy  the  outer  regions.  Fiber-reinforced 
composites  can  have  specific  strengths  and  moduli  up  to  six  times  those  of  common 
structural  materials.  For  a  given  weight,  fiber-reinforced  composites  far  outperform 
other  engineering  materials  in  their  strength  and  stiffness.  These  specific  strengths  and 
moduli  approach  the  mechanical  properties  of  theoretically  perfect,  ordered  polymer 
crystals.  This  property  makes  composite  materials  unique  among  engineering  struc- 
tural materials  and  opens  new  horizons  for  novel  engineering  designs.  For  example, 
composite  materials  are  widely  used  in  aircraft  and  aerospace  applications:  The  FRP 
property  of  high  specific  strength  with  high  elasticity  modulus  made  possible  the  de- 
sign, construction,  and  functional  deployment  of  the  U.S.  Air  Force  all-carbon  fiber- 
reinforced  epoxy  resin  composite  Stealth  reconnaissance  aircraft. 

The  observed  high  strength  and  stiffness-to-weight  ratio  of  fiber-reinforced  com- 
posites can  be  easily  explained.  Various  material  properties  of  composites  can  be  esti- 
mated by  a  rule  of  mixtures  approach.  Micromechanic  properties  such  as  modulus 
(stiffness),  strength,  Poisson's  ratio,  and  thermal  expansion  of  fiber-reinforced  polymer 
composites  can  be  estimated  by  the  following  equation: 


Mc  =  VfMt  +  VmMn 


(8.1) 


306 


8      Plastics 


FIGURE    8.21 

Specific  strengths  and 
specific  elastic  moduli 
of  materials 


1   ' 

Key 

' 

'       ' 

UT77\ 

Composite  (unidirectional) 

Gr 

Graphite 

M 

Metals 

Gl 

Glass 

P 

Plastics 

B 

Boron 

E 

Elastomers 

K 

Kevlar 

HM 

High  Modulus 

Ep 

Epoxy 
HTGr 

HT 

High  Strength   — 

Gl 

o 

o 

HMGr 

— 

o 

— 

K/Ep 

B    _ 

~   xmn 

o 

>x     (HP 

■*—    B/Ep 

.  Gl/Ep 

Gr/Ep 

/ 

yS 

-<UD 

Chopped      \^ 

— ^-^y^^ffiPtx 

^^Gr/Ep 

— 

(py           r^l^vr 

©W  |          \^ 

i_                    _L 

| 

| 

200  400  600 

Specific  modulus,  inches  x  10-6 


800 


where:  M  is  the  particular  material  property 

V  is  the  volume  fraction  of  the  fiber  (f )  or  matrix  (m) 
Mc  is  the  material  property  of  the  composite  "mixture" 

The  individual  material  component  properties,  therefore,  contribute  by  a  volume  frac- 
tion ratio  to  the  properties  of  the  combined  composite  materials.  For  this  rule  of  mix- 
tures equation  to  apply,  several  basic  assumptions  and  limitations  are  involved: 

1.  The  fiber/polymer  matrix  composites  as  well  as  the  polymer  matrix  are  assumed 
to  be  linearly  elastic  and  homogeneous. 

2.  There  are  no  voids  in  the  composite,  and  there  is  good  adhesion  between  the  rein- 
forcing fibers  and  the  polymer  matrix. 

3.  The  proximity  of  the  fiber  and  polymer  does  not  alter  the  properties  of  the  indi- 
vidual components. 

4.  The  rule  of  mixtures  has  some  directional  limitations  as  many  FRPs  are  not 
isotropic  materials. 


8.5  Fiber-Reinforced  Polymeric  Composites  307 

For  example,  if  we  are  dealing  with  a  continuous  fiber-reinforced  polymer  resin 
composite,  the  modulus  and  strength  properties  of  the  composite  will  be  very  different 
in  the  direction  longitudinal  to  the  fiber  length  compared  to  the  properties  across  or 
perpendicular  to  the  fibers.  For  strength  and  modulus,  Equation  8.1  is  most  appropri- 
ate for  composites  being  tested  in  the  longitudinal  (fiber)  direction.  The  mechanical 
contribution  of  the  fibers  are  directly  in  line  with  the  direction  of  pull.  The  fibers  are 
strong  and  stiff  in  this  longitudinal  direction,  and  the  polymer  matrix  is  relatively  weak 
and  much  less  rigid.  Note  that  the  strength  and  stiffness  of  materials  in  fiber  form 
are  always  much  higher  than  bulk  materials  (e.g.,  bar,  rod,  plate)  because  the  fiber 
form  of  a  material  has  a  more  atomically  ordered  internal  structure.  Fibers  have  an 
internal  crystalline  structure  that  favorably  alters  the  stiffness  and  fracture  behavior 
of  this  form  of  material.  The  presence  of  fibers  makes  composites  stiffer  and  stronger 
in  the  longitudinal  (fiber)  direction  than  the  polymer  matrix  by  itself.  The  term  fiber- 
reinforced  polymer  is  thus  appropriate.  Property  directionality  effects  are  very  impor- 
tant to  consider  in  the  use  of  fiber-reinforced  composites  in  engineering  designs. 

Fiber  Reinforcement 

Generally,  reinforcement  in  FRPs  can  be  either  fibers,  whiskers,  or  particles.  In  composite 
materials  of  the  most  commercial  interest,  fibers  are  the  most  important  and  have  the  most 
influence  on  composite  properties.  Table  8.3  presents  a  comparison  of  the  most  common  re- 
inforcement fibers  used  in  preparing  organic  polymer  engineering  composites.  Nylon  fiber 
is  included  here  as  areference  fiber.  All  the  materials  listed  in  Table  8.3  are  textile  fibers  and 
can,  for  the  most  part,  be  processed  into  manufactured  products  in  the  same  manner  as  tex- 
tile fibers  (e.g.,  continuous  yarn,  wound  filaments,  woven  and  knitted  fabrics,  nonwoven 
mats).  The  high-strength  and  high-stiffness  properties  of  the  glass  (S-2),  carbon,  and  pol- 
yaramid  fibers  are  evident.  These  reinforcing  fibers,  when  used  in  composite  material  fab- 
rication, can  take  several  forms,  such  as  <0. 1  inch  (3^4-  mm)  fiber  "whiskers,"  0. 1  -0.3  inch 
(3-10  mm)  chopped  fibers,  0.1-2.0  inch  (3-50  mm)  (nonwoven)  matted  fiber  sheets, 
woven  fabric  (continuous)  fiber  with  plain  weave,  and  unidirectional/longitudinal  (contin- 
uous) fiber  ribbons.  These  fiber  reinforcement  forms  are  illustrated  in  Figure  8.22.  When 
using  fiber  reinforcement  in  polymer  composites,  the  surface  of  the  fibers  or  yarns  is  pre- 


TABLE    8.3 

Comparison  properties  of  various  fibers 


S-2  Glass 

Carbon  T-300 

Aramid-49 

Nylon  6/6 

Tensile  Strength, 

665,700 

390,100 

400,000 

143,000 

lb/in.2  (MPa) 

(4,590) 

(2,690) 

(2,758) 

(2,758) 

Modulus  of  Elasticity, 

13,000,000 

33,000,000 

18,000,000 

800,000 

lb/in.2  (MPa) 

(87,000) 

(229,000) 

(124,110) 

(5,516) 

Elongation,  % 

5.4 

1.2 

2.5 

18.3 

Density, 

0.090 

0.062 

0.052 

0.041 

lb/in.3  (g/cm3) 

(2.49) 

(1.73) 

(1.44) 

(1.14) 

308 


8      Plastics 


FIGURE    8.22 

Comparison  of  fiber 
reinforcement  forms 


\     s    N     '      '       "       S--        \      ^    X     ' 

'        ' 

~ '  .",  -           x  '  \  ~~ '    ',  - 

N       ~" 

/ 
1 

-.       1       /    N    '      ^        -     __     -       1       / 

\     / 

/ 

,       V                /       -        v                                -                 ^        X 

~      • 

Whiskers 
<3-4  mm 


Chopped  fibers 
3-1 0  mm 


Fiber  (non-woven)  mat 
>3-50  mm 


iff 


mm 


TTUTTUTT 


Woven  fabric 
continuous 


Parallel  aligned  yarns 
continuous 


8.5  Fiber-Reinforced  Polymeric  Composites  309 

treated  with  a  chemical  coupling  agent  to  enhance  wetting  and  adhesion  of  the  matrix  resin 
to  the  fibers.  Here,  the  chemical  coupling  agents  are  made  specific  to  the  chemical  nature  of 
the  matrix  resin  being  used.  It  is  important  that  the  fiber  supplier  be  consulted  for  the  proper 
type  of  fiber/resin  coupling  agent  when  fiber  reinforcement  materials  are  purchased. 

As  Table  8.3  shows,  the  most  commonly  used  reinforcing  fiber  material  is  glass. 
In  particular,  S-2  glass  is  used  in  most  high-performance  applications.  There  exists  an 
extensive  applications,  manufacturing,  and  processing  history  involving  the  use  of  glass 
fiber  in  polymer  composite  applications.  Various  forms  of  carbon  fiber  are  also  used 
for  high-performance  applications.  The  processing  of  carbon-fiber-reinforced  polymer 
composites  follows  similar  procedures  as  glass  fibers.  However,  in  the  continuous-yarn 
processing  of  carbon  fibers,  precautions  must  be  taken  to  protect  electrical  processing 
equipment  from  damage.  Airborne,  electrically  conducting  graphite  dust  is  generated 
when  the  carbon  fibers  or  yarns  are  processed  through  guide  rings  and  rollers.  This  can 
occur  before  the  fibers  are  wetted  by  the  matrix  resin  during  material  fabrication.  The 
dust  can  ruin  electrical  equipment  if  it  is  allowed  to  penetrate  an  instrument's  enclo- 
sure. Sometimes,  explosion  proof  electrical  equipment  is  used  when  processing  carbon 
fibers.  Another  approach  is  to  fit  the  electrical  instrument  housing  with  a  positive  pres- 
sure differential  of  clean  air  (or  nitrogen  gas). 

Matrix  Resins 

Classification  of  polymer  matrices.  Many  types  of  polymers  and  resins  can  be  rein- 
forced by  fibers  to  create  FRP  composite  materials.  Polymer  matrices  can  be  classified 
into  two  basic  categories:  thermoplastic  and  thermosetting. 

1.  Thermoplastic.  Many  of  the  polymers  previously  discussed  can  be  reinforced  with 
fibers  to  form  composites.  The  most  common  types  are  chopped-fiber-reinforced 
thermoplastics.  These  materials  can  be  processed  in  the  same  way  as  nonfiber- 
reinforced  plastics.  Generally,  chopped  fibers  are  blended  and  mixed  into  a  molten 
mass  of  the  engineering  thermoplastic  (e.g.,  nylon,  polycarbonate,  acetal)  in  a  melt- 
extruder  type  of  plastics-compounding  machine.  The  fiber  containing  plastic  is  ex- 
truded into  a  thin  rod  and  cut  into  molding  powder  or  pellets.  This  thermoplastic 
molding  powder  is  then  used  for  injection  molding  or  extrusion  of  engineered  parts 
similar  to  the  unreinforced  plastics  discussed  in  the  preceding  sections. 

Continuous  fibers  such  as  glass,  carbon,  or  polyaramid  have  also  been  prepared 
with  thermoplastic  resin  matrices.  The  concept  here  is  to  first  coat  thermoplastic 
resins  onto  continuous-fiber  yarn  by  a  hot  melt  or  a  polymer  solution-solvent-dip 
process.  These  thermoplastic  polymer-coated  yarns  can  then  be  fabricated  into 
shaped  structures  by  a  (hot  press)  matched-die  compression  molding  technique  or 
other  techniques  for  affecting  molten-polymer  controlled  consolidation.  At  this 
time,  discontinuous  chopped-fiber  thermoplastic  composites  are  much  more  widely 
used  than  continuous  fiber-reinforced  composites.  The  main  advantages  of  thermo- 
plastic matrix  fiber  composites  is  that  they  can  be  processed,  for  the  most  part,  in 
conventional  thermoplastic  polymer  fabrication  equipment.  Furthermore,  any  scrap 
or  off-quality  material  can  be  recycled  back  into  the  injection  molding  or  extruding 
machine.  However,  care  must  be  taken  during  this  thermoplastic  processing  not  to 


310  8      Plastics 


overly  "work"  these  materials  in  the  molten  state.  Excessive  processing  in  the 
molten  state  severely  shortens  the  overall  reinforcing  fiber  length,  which  can  di- 
minish the  reinforcement  effect  of  the  fiber  in  the  polymer  matrix. 

2.  Thermosetting.  Reinforced  composites  are  traditionally  associated  with  thermoset- 
ting polymers  such  as  the  unsaturated  polyester  and  epoxy  resins.  In  their  cured 
state,  thermosetting  resins  are  composed  of  long  polymer  chains  that  are  joined  to- 
gether through  cross-bridges  that  link  together  all  the  molecules  in  the  resin  mass. 
The  final,  hardened,  tough,  and  glassy  state  of  the  cured  resin  is  the  terminal  con- 
dition of  the  polymer  resin  matrix.  In  this  state,  the  resin  serves  the  all-important 
role  of  structurally  consolidating,  supporting,  and  cohesively  tying  together  the 
fiber  reinforcement  in  the  composite.  However,  during  initial  processing,  it  is  im- 
portant that  thermosetting  resins  undergo  a  gradual  liquid-to-solid  conversion.  It  is 
this  feature  that  renders  thermosetting  resins  of  the  unsaturated  polyester  and  epoxy 
type  most  readily  adaptable  to  fiber-reinforced  composite  component  fabrication. 

Sequence  of  FRP  fabrication  with  respect  to  the  resin  system  involved.  At  first,  the 
resin  is  in  a  liquid  state  as  it  is  received  from  the  supplier.  It  may  be  more  or  less  fluid 
depending  on  its  viscosity  (from  a  flowable  waterlike  consistency  to  a  high-viscosity 
syrup).  At  this  stage,  rheological  thickeners  to  increase  resin  viscosity  or  reactive  dilu- 
ents to  decrease  resin  viscosity  may  be  added  to  the  resin  formulation.  Frequently,  the 
curative  part  of  the  resin  system  is  much  more  fluid  than  the  resin  part.  Here,  the  vis- 
cosity of  the  final  mixed  resin  and  curative  system  is  low  enough  to  accommodate 
proper  flow  in  processing.  Sometimes,  the  fluidity  of  the  resin  may  be  lowered  by  in- 
creasing the  temperature  of  the  resin  upon  its  application  to  the  fiber.  In  all,  it  is  im- 
portant that  the  viscosity  of  the  liquid  resin  be  adjusted  so  that  it  has  the  proper  fluidity 
to  wet,  impregnate,  and  saturate  the  reinforcing  fiber  yarns,  fabric,  or  mat. 

The  next  consideration  is  the  need  to  chemically  catalyze  the  resin  so  that  it  prop- 
erly cross-links  and  cures  the  resin  under  the  prescribed  conditions.  It  is  also  necessary 
to  have  the  catalyzed  resin  react  very  slowly  at  ambient  temperature  so  that  it  remains 
fluid  while  it  is  in  the  process  of  wetting  the  reinforcing  fibers.  This  resin-system  fluid 
time  is  referred  to  as  the  pot  life  or  open  time  of  the  resin.  This  fluid-time  feature  is 
controlled  by  the  nature  of  the  catalyst,  the  ambient  temperature,  and  the  bulk  volume 
of  resin  in  the  resin  container.  Note  that  a  bulk  of  catalyzed  resin  is  a  resin  undergoing 
a  heat-generating  exothermic  reaction.  If  the  bulk  volume  of  the  resin  is  too  large,  heat 
cannot  be  easily  dissipated.  The  reaction  in  the  fiber/resin  dip  tank  will  automatically 
accelerate,  the  resin  will  cure,  or,  worse,  the  heat  of  the  reaction  may  cause  a  serious 
fire  as  well  as  noxious  fumes.  Most  often,  however,  the  processing  equipment  will  con- 
tain dual-component  pumps  and  a  mixing  head  that  will  continuously  meter  and  mix 
the  proper  components  of  the  resin  system  (resin:  part  A;  curative:  part  B)  at  the  ap- 
propriate moment  and  position  for  wetting  the  fibers. 

Once  the  resin  part  and  the  curative  part  of  the  resin  system  have  been  mixed,  the 
liquid-to-solid  cure  reaction  of  the  resin  begins.  The  curing  resin  system  will  undergo 
several  stages:  liquid/fluid,  gel  stage,  rubbery  stage,  and  tough/glassy  solid.  Depending 
upon  the  processing  temperature,  the  liquid-to-gel-to-rubber  transition  may  occur  from 
hours  (for  room  temperature  cures),  to  minutes,  to  seconds.  The  gel  point  in  a  ther- 


8.5  Fiber-Reinforced  Polymeric  Composites 


311 


mosetting  resin  system  is  the  point  in  the  cure-time  sequence  when  the  resin  undergoes 
a  sharp  rise  in  viscosity  and  ceases  to  be  a  fluid.  Theoretically,  the  gel  point  is  defined 
as  the  time  in  the  cure  when  each  polymer  molecule  in  the  system  is  tied  together  by 
at  least  one  cross-link.  Therefore,  at  the  gel  point,  the  polymer  molecules  in  the  resin 
system  have  combined  and  have  reached  an  infinite  molecular  weight.  After  the  gel 
point,  the  number  of  cross-links  in  the  polymer  system  continues  to  increase,  the  cross- 
link network  gets  tighter  and  tighter,  and  the  resin  becomes  a  solid.  It  is  at  the  gel  stage 
that  the  influence  of  cross-linking  takes  hold.  The  rubbery  stage  is  intermediate  in 
cross-linking.  In  the  solid  glassy  state  of  the  resin,  the  ultimate  number  of  cross-links 
in  the  resin  system  exists.  Figure  8.23  illustrates  the  nature  of  the  polymer  resin  and 
curative  molecules  during  the  curing  sequence.  Note  that  it  is  only  in  the  solid-state 
stage  that  the  fabricated  composite  part  retains  its  shape  and  may  be  moved  for  addi- 
tional processing  or  given  a  postcure  if  necessary.  Let  us  now  examine  the  specific 
resin  chemistry  of  the  unsaturated  polyester  and  epoxy  resin  systems. 

Chemistry  of  the  unsaturated  polyester  resin  system.  Unsaturated  organic  polymers 
are  polymer  systems  containing  double  bonds,  or  C  =  C.  Double  bonds  can  react  with 
each  other  by  an  addition  reaction  that  can  be  initiated  by  a  free-radical  catalyst.  With 
the  help  of  free-radical  catalysts,  unsaturated  organic  compounds  can  react  with  each 
other  to  form  high-molecular-weight  polymers: 


"H 

l-T 

C  = 

=  C 

_R 

R'_ 

unsaturated 
monomer 


catalyst 


bond  opening 


r                      i 

3D O I 

1 

H 
p 

R' 

polymer 


In  unsaturated  polyester  resins,  the  resin  part  of  the  mixture  is  represented  by  high- 
molecular-weight  polymer  molecules  having  unsaturated  groups  in  their  chain.  These 
unsaturated  polyesters  are  readily  soluble  in  the  unsaturated  organic  liquid  compound 
called  styrene.  Styrene  (known  here  as  a  monomer)  can  easily  react  with  itself  (using  a 
free-radical  catalyst  initiator)  to  form  a  styrene  polymer,  or  polystyrene.  Because  the 
monomeric  styrene  can  readily  react  with  unsaturated  groups,  when  liquid  styrene  is 
mixed  with  unsaturated  polyester  resin,  it  serves  the  dual  role  of  a  reactive  diluent  and 
cross-linking  agent.  If  a  free-radical  catalyst  is  added  to  a  solution  mixture  of  unsatu- 
rated polyester  resin  and  styrene,  the  styrene  simultaneously  reacts  with  both  the  unsat- 
uration  in  the  backbone  of  the  polyester  chain  and  with  itself.  With  free-radical  catalysis, 
the  polymerization  reaction  involving  the  growing  polystyrene  chains  that  react  with  the 
one  polyester  chain  can  also  react  with  itself.  When  this  reaction,  in  turn,  connects  with 
another  polyester  chain,  a  cross-link  is  formed  between  the  two  chains.  In  the  molecu- 
lar mixture  mass  of  styrene,  growing  polystyrene  chains,  and  unsaturated  polyester  mol- 
ecules, an  array  of  cross-links  are  formed  between  the  multitude  of  polyester  molecules 
(see  Figure  8.24).  As  the  polymer  system  reacts,  from  its  initial  mixing  of  the  catalyst, 
the  resin  system  will  change  from  a  liquid,  to  a  gel-rubber  when  cross-linking  starts  to 


312 


8      Plastics 


FIGURE    8.23 

Nature  of  molecules  at 
various  stages  of 
thermosetting  resin 
cure 


BACKBONE  Polymer  (Pre-Polymer) 

CROSSLINICING       Chemical/Agent 


Liquid  -  All  molecules 
are  independent, 
can  flow  past  each 
other 


Gel  -  At  least  one 
crosslink  attachment 
to  each  backbone 
ploymer  molecule. 


Rubber  -  More 
crosslinks,  backbone 
still  flexible 


•  Glass  -  High 
crosslink  density, 
tight  network 
structure 


8.5  Fiber-Reinforced  Polymeric  Composites 


313 


FIGURE    8.24 

Chemical  structure  of 
unsaturated 
polyester/styrene  resin 


H        H 

•  H 

<{>         H 


Unsaturated 

polyester  resin 

(UP) 


Styrene 
(Monomer) 


H        H        H        H        H 
/™  —  c  —  C  —  C  —  C  —  C'™ 

(j)        H        (j>        H        (J) 


H        H     \ 

1          1       \ 

Polystyrene 

c  — c  — 

(Polymer) 

1          1 

(PS) 

<t>        H     In 

PS 


PS 


-</WN      C    C    /WWWW\    Q    Q 

c       Crosslinks 


UP/PS 

Crosslinked 
polymer 

resin 
network 


occur,  and,  finally,  to  a  solid  glassy  vitreous  state  when  numerous  cross-links  form  and 
tie  together  the  polyester  molecules  in  the  resin  system.  This,  in  essence,  is  the  chemi- 
cal mechanism  that  characterizes  the  cure  of  a  typical  polyester  resin. 

In  the  commercial  formulation  of  unsaturated  polyester/styrene  thermosetting  FRP 
resins,  to  make  the  resin  more  sag  resistant  when  applied  to  vertical  and  more  con- 
toured surfaces,  fumed  silica  is  added  to  alter  the  rheology  of  the  liquid  resin.  Another 
additive  involves  using  a  wax  material  that  serves  as  a  surface  active  agent  that  allows 
the  resin  to  cure  more  evenly  at  its  surface.  Unsaturated  polyester  resin  systems  are  by 
far  the  most  widely  used  FRP  matrix  resins  because  of  their  low  cost  and  availability. 
However,  their  use  is  being  questioned  because  of  environmental  concerns.  Styrene 
monomer  is  quite  odiferous,  and  questions  are  being  raised  regarding  its  human  toxic- 
ity after  long-term  process-operator  exposure. 

Chemistry  of  the  epoxy  resin  system.  Because  of  their  inherent  good  adhesion  to  all 
types  of  surfaces,  epoxy  resins  are  generally  more  difficult  to  work  with  than  poly- 
esters. However,  epoxies  have  much  better  thermal  properties  and  exhibit  very  low 
shrinkage  during  cure.  Their  adhesive  properties,  while  adding  process  difficulties, 
serve  to  enhance  the  structural  integrity  of  the  fiber/resin  composite  material  system. 
Epoxies  provide  good  adhesion  of  the  resin  matrix  to  the  reinforcing  fibers.  The  major 
hardeners  for  epoxy  resins  are  amines  and  anhydrides.  The  chemistry  of  these  hard- 
ener/curative systems  is  discussed  next. 


314  8      Plastics 


Epoxy  resins  are  characterized  by  the  reaction  of  the  epoxy  group  c c 

known  as  the  oxirane  ring.  Polymerization  reactions  proceed  by  the  opening  of  this 
oxirane  ring  to  form  a  difunctional  chemical-reacting  specie  similar  to  the  unsaturated 
C  =  C  group  in  polyesters.  Epoxy  resins  are  low-molecular- weight  polymers  contain- 
ing oxirane  rings  at  each  end  of  the  chain.  They  are  cured  by  adding  a  multifunctional 
chemical  to  the  mixture  that  serves  to  cross-link  the  system  by  an  addition  reaction 
with  the  oxirane  ring.  The  most  common  cross-linking  agents  for  epoxies  are  the 
amines.  Many  of  the  amines  used  to  cure  epoxies  are  liquids,  which  makes  the  amines 
serve  as  reactive  diluents.  Such  liquid  material  systems  are  also  easily  adaptable  to 
dual-component  pumps  and  the  mixing  of  resin  during  dispensing  for  processing.  The 
basic  reaction  between  (primary)  amine  groups  and  the  epoxy  group  is  as  follows: 

OH 


RNH, +  CH 


1?  — 

—  CH *~  RNH CH2 CH 

?H          o 

/  \ 

CH? 

CH  +  CH2 CH >-  RN  (CH2 

RNH  CH2 CH  +  CH2 CH ►  RN  (CH2 CHOH) 


As  shown,  each  of  the  two  hydrogen  atoms  of  the  primary  amine,  RNH2,  where  R  is  a 
generic  unspecified  organic  grouping,  is  capable  of  reacting  with  one  epoxide  group.  In 
this  chemical  process,  as  with  polyester  resins,  the  epoxy  polymer  passes  from  a  liquid  to 
a  gel-rubber  to  the  solid  glassy  state  as  the  cross-linking  reaction  proceeds.  During  the  lat- 
ter stages  of  the  reaction,  the  resultant  OH  groups  that  are  formed  in  the  amine  reaction  can 
also  react  with  epoxy  groups  and  further  increase  the  cross-link  density  of  the  polymer. 

One  advantage  of  amine-cured  epoxy  resins  is  that  they  can  harden  or  cure  at 
room  temperature.  However,  room  temperature  curing  leads  to  polymers  with  low 
temperature  stability.  Also,  the  moisture  resistance  of  these  epoxy  resins  is  generally 
low.  Both  temperature  and  moisture  resistance  can  be  improved  by  postcuring  the 
resins  above  212°F  (100°C).  Here,  the  chemical  cross-links  of  the  resin  are  maximized 
as  complete  reaction  of  all  the  epoxy  groups  is  approached. 

The  reaction  of  anhydride  curing  agents  with  epoxy  resins  is  more  complex  than 
that  of  amine  cures.  With  anhydrides,  amine  catalysts  are  required  along  with  cures  at 
high  temperature.  During  reaction,  several  competing  reactions  can  take  place.  The 
most  important  reactions  are  as  follows: 

1.  Opening  of  the  anhydride  ring  with  the  OH  groups  from  the  catalytically  reacted 
epoxy  groups  to  form  a  carboxyl  group: 

o 
c c^  !  c c o CH 

O      +      HO CH       *-  : 

— c — c  i  — c — c — OH 

I      V 


o 


o 


anhydride  epoxy  resin  reaction  product 

moiety 


8.5  Fiber-Reinforced  Polymeric  Composites  315 

2.  Subsequent  reaction  of  the  carboxyl  group  with  the  epoxy  group: 

o  o 

■  o 

C C O CH  /    \  C C O CH 

i     +   CH2 — CH •     — *~  i 

C C OH  C C O CH2 CH  

O  O  OH 

3.  Epoxy  groups,  in  turn,  reacting  with  the  formed  OH  groups: 

!  /°\ 

HC OH     +     CH2 CH ►■  = 

=  HC O CH? CH  


Although  all  three  reactions  can  occur,  which  of  the  three  reactions  predominates  de- 
pends on  the  reaction  temperature. 

Compared  to  amine  cures,  the  pot  life  of  anhydride  cures  is  long,  and  the  reaction 
produces  a  low  exotherm.  Long-time,  elevated-temperature  cures  up  to  392°F  (200°C) 
are  necessary  if  ultimate  properties  are  desired.  Overall,  compared  to  amine-cured  sys- 
tems, anhydride  cures  result  in  much  better  chemical  resistance  for  the  final  cured 
resin  product. 

From  a  processing  standpoint,  the  environmental  and  industrial  hygiene  aspects  of 
amine-  or  anhydride-cured  epoxy  resins  are  much  better  than  the  hygiene  problems 
associated  with  unsaturated  polyester  resin  processing.  In  all  cases,  proper  protective 
clothing  (coat,  gloves,  and  goggles)  must  be  worn  while  working  with  these  resins. 
Amine  and  anhydride  chemicals  are  generally  quite  corrosive  to  the  skin  and  may  cause 
dermatitis. 

Forms  of  Composite  Materials 
and  Fabrication  Techniques 

Discontinuous  fiber  reinforcement.  The  reaction  injection  molding  (RIM)  process  in- 
volves bringing  together  two  components  of  a  thermosetting  polymeric  resin  system  in 
a  mixing  head  and  injecting  the  reacting  mixture  into  a  closed  mold  before  reaction  is 
complete,  as  illustrated  in  Figure  8.25.  The  resin  system  then  cures  in  the  mold  at  a  rel- 
atively low  pressure  of  50  psi  (345  kPa).  The  timing  of  the  curing  reaction  is  very  im- 
portant because  the  reaction  must  occur  at  the  moment  the  mold  cavity  is  filled.  Close 
process  control  is  required.  Because  the  process  involves  low-viscosity  intermediates, 
complex  parts  can  be  fabricated  using  the  RIM  method. 


316 


8      Plastics 


FIGURE    8.25 

The  reaction  injection 
molding  (RIM)  process 


Resin 
component  A 


Resin 
component  B 


High- 
pressure 
^    cylinder 


Reinforcement  (glass,  fiber,  or  flake)  can  be  added  to  one  of  the  resin  components 
prior  to  mixing  if  increased  flexural  modulus,  thermal  stability,  and,  in  some  instances, 
a  special  surface  finish  is  desired  in  the  final  molded  product.  This  process,  reinforced 
reaction  injection  molding  (RRIM),  is  shown  in  Figure  8.26. 

Structural  reaction  injection  molding  (SRIM)  and  resin  transfer  molding  (RTM) 
are  similar  to  RRIM,  except  that  the  reinforcement  is  placed  directly  into  the  mold 
prior  to  the  injection  of  the  resin.  In  SRIM,  the  reinforcement  is  typically  a  preform  of 
reinforcement  fibers  or  mat  of  nonwoven  fibers.  In  RTM,  as  shown  in  Figure  8.27,  a 
catalyzed  resin  is  pumped  directly  into  the  mold  cavity  containing  the  reinforcement. 


FIGURE    8.26 

The  reinforced  reaction 
injection  molding 
(RRIM)  process 


Reinforcement 

Resin 
component  A 


Resin 
component  B 


High-pressure 
cylinders 


Mold 


8.5  Fiber-Reinforced  Polymeric  Composites 


317 


FIGURE    8.27 

The  resin  transfer 
molding  (RTM)  process 


Resin 


Dry  Reinforcement 


The  resin  system  is  such  that  it  cures  without  heat.  The  advantages  of  RTM  are  that, 
because  no  mixing  head  is  involved,  a  relatively  low  investment  is  needed  for  equip- 
ment and  tooling.  Furthermore,  large  FRP  parts  and  parts  containing  inserts  and  cores 
can  be  fabricated  using  the  RTM  process.  RIM,  RRIM,  SRIM,  and  RTM  processing 
are  widely  used  in  the  automotive  and  aerospace  industries. 

Wet  lay-up  and  vacuum  bagging.  Imbedding  plies  of  glass,  carbon,  and/or  polyaramid 
plain-weave  fabric  or  fibrous  mat  into  an  uncured  liquid  resin  and  allowing  the  liquid 
resin  to  solidify  (cure)  while  being  constrained  by  a  mold  or  form  is  a  common  pro- 
cessing technique  used  in  the  pleasure  boat  building  industry.  A  typical  arrangement  of 
the  plies  used  in  this  technique,  called  the  wet  lay-up  process,  is  shown  in  Figure  8.28. 
Related  to  this  wet  lay-up  process  is  the  vacuum  bagging  method  of  fabricating  com- 
posite parts  and  shapes.  The  principle  of  vacuum  bagging  is  quite  simple.  The  shape  to  be 
fabricated  is  prepared  by  a  room  temperature  wet  lay-up  procedure  as  just  described.  The 
part  to  be  fabricated  is  usually  assembled  over  a  form  or  shape  of  the  desired  (complex 
and/or  contoured)  part.  The  assembly,  like  the  lay-up  arrangement  shown  in  Figure  8.28, 
is  then  placed  in  an  airtight  disposable  plastic  "bag"  fitted  with  a  vacuum  tube  fitting  or 
stem.  If  the  air  is  sealed  off  and  then  evacuated  from  it,  the  bag  will  automatically  close  in 
on  the  wet  laid-up  plies  of  fiber  and  liquid  (uncured)  resin  and  consolidate  these  plies  by 


FIGURE    8.28 

Arrangement  of  plies  in 
wet  lay-up  assembly 


u  a  d  n  u  u  u  a  u  u  u  u  o~q 
n  n  n  n  n  n  w  n  n rrri  n  n  rn 


VBF  -  Impermeable  vacuum  bag  film 

B  -  Conformable  nonwoven  bleeder/breather  fabric 

P  -  Perforated  release  film 

C  -  Fiber  reinforced  resin  composite  part 

S  -  Pressure  sensitive  flexible  sealant 


318 


8      Plastics 


the  action  of  atmospheric  pressure.  This  composite  assembly  is  then  allowed  to  solidify 
(cure)  at  room  or  elevated  temperature.  After  this  cure  time,  the  vacuum  bag,  bleeder 
ply,  and  resin-absorber  material  are  removed  from  the  assembly  and  discarded,  leaving 
the  fabricated  composite  part  ready  for  subsequent  finishing  or  treatment. 

A  variation  of  the  wet  lay-up  method  is  the  spray-up  process,  where  a  spray  gun  si- 
multaneously sprays  catalyzed  resin  and  chops  continuous  glass  yarn  into  specific 
lengths.  As  shown  in  Figure  8.29,  chopped  fibers  enter  the  spray  nozzel  of  the  spray  gun, 
and  the  materials  are  comixed  and  sprayed  onto  an  open-cavity  mold.  The  mold  usually 
is  faced  with  a  smooth  coating  of  already  cured  resin  called  a  gel-coat  or  a  thermoplas- 
tic shell.  This  forms  the  outer  surface  of  the  structure  being  fabricated.  When  the 
sprayed-on  fiber-reinforced  resin  cures,  the  part  is  removed  from  the  mold.  The  laminar 
structure  formed  is  composed  of  an  aesthetically  acceptable  or  otherwise  finished  outer 
skin.  Adhered  to  and  backing  up  this  skin  is  the  cured  fiber-reinforced  resin.  Open-mold 
processing  of  this  type  is  used  extensively  in  bathtub  and  shower  stall  applications. 

Unidirectional-fiber  resin  prepregs.  Fiber-reinforced  composite  materials  are  com- 
monly used  in  the  form  of  a  prepreg.  Prepregs  are  typically  side-by-side  aligned  fiber 
yarns  that  have  been  impregnated  by  a  B-staged  resin  matrix  (meaning  that  it  has  been 
deliberately  partially  cured).  Unidirectional-fiber  composite  prepregs  are  commercially 
available  in  the  form  of  rolls,  tapes,  and  sheets.  One  drawback  is  that  these  prepregs 
must  be  kept  frozen,  below  32°F  (0°C),  for  shipping  and  storage  before  use.  They  also 
have  a  relatively  short  shelf  life.  If  not  properly  stored,  the  B-stage  resins  will  cure 
slowly  at  room  temperature,  and  their  function  will  be  destroyed. 

Prepreg  material  is  used  to  fabricate  structures  by  plying  together  lay-ups  of  these 
resin-impregnated  unidirectional  fibers.  The  lay-ups  can  be  designed  to  have  different 
desired  mechanical  properties  depending  upon  the  geometrical  arrangement  or  assem- 
bly of  the  reinforcing  fibers  in  the  cured  lay-up.  Some  typical  unidirectional-fiber  ply 
arrangements  are  shown  in  Figure  8.30.  Mechanically,  these  unidirectional  (0°,  0°), 
cross-ply  (0°,  90°),  and  quasi-isotropic  (0°,  +45°,  90°,  -45°,  0°)  plied  laminates  will 


FIGURE    8.29  ^^ 

The  spray-up  process  f  J  \ — 


8.5  Fiber-Reinforced  Polymeric  Composites 


319 


FIGURE    8.30 

Various  arrangements 
of  unidirectional-fiber 
ply  laminates 


Unidirectional 
(0°,0°) 


Cross-ply 
(0°,90°) 


Quasi-isotropic 
(0°,+45o,-45o,90°) 


have  planar  anisotropic  properties.  Their  flexural  stiffness  will  always  be  higher  in  the 
longitudinal  direction  of  the  fibers.  Other  forms  of  B-stage  resin-impregnated  fiber 
forms  are  commercially  available  (e.g.,  fabrics  and  fibrous  mats).  The  numerous  B- 
stage  precomposite  forms  and  types  of  fiber  are  all  available  to  the  composite  materi- 
als design  engineer  in  the  construction  of  a  fiber-reinforced  composite  structure. 


320  8      Plastics 


Filament  winding.  Filament  winding  is  a  fiber-reinforced  composite  processing  pro- 
cedure commonly  used  to  fabricate  tubular  (hollow)  and  cylindrical  tank  or  bottle- 
like structures.  The  apparatus  used  in  the  filament  winding  process  is  shown  in  Figure 
8.31a.  Basically,  filamentary  yarns  are  fed  off  a  spool  that  is  mounted  on  a  creel.  The 
yarn  is  immersed  in  a  catalyzed,  but  still  liquid,  resin  bath,  where  the  yarn  is  impreg- 
nated with  the  resin.  After  squeezing  out  excess  resin,  the  resin-impregnated  yarn  is 
wound  onto  a  rotating  mandrel  in  a  controlled  and  directed  manner.  A  computer  sys- 
tem and  control  arm  guide  the  yarn  back  and  forth  across  the  mandrel  in  a  predeter- 
mined pattern.  The  computer  controls  the  type  of  wind  pattern  and  the  number  of 
layers  of  yarn  filaments  to  be  laid  down  on  the  mandrel  surface.  Two  types  of  wind 
patterns  are  possible:  circumferential  and  helical,  (as  Figure  8.31b  shows).  In  the  cir- 
cumferential or  hoop  wind,  the  yarn  is  wound  in  a  continuous  manner  in  close  prox- 
imity alongside  itself.  No  crossover  of  the  yarn  occurs  during  the  lay-down  of  a  given 
layer,  and  the  lay-down  pattern  can  thus  be  considered  to  be  at  a  zero  wind  angle.  The 
wind  proceeds  back  and  forth  across  the  mandrel  until  the  desired  number  of  layers  is 
accomplished.  In  the  helical  wind,  the  yarn  is  permitted  to  cross  over  itself  and  tra- 
verses the  length  of  the  mandrel  at  a  prescribed  angle  (e.g.,  10°,  30°,  45°).  Again,  the 
wind  proceeds  back  and  forth  across  the  surface  of  the  rotating  mandrel  until  the  de- 
sired number  of  layers  is  formed. 

In  practice,  combinations  of  hoop  and  helical  wind  are  usually  performed  to  fab- 
ricate a  part.  The  desired  lay-down  sequence  is  programmed  on  the  computer.  While 
the  desired  (yarn)  filament-wound  resin  composite  is  being  formed  on  the  mandrel, 
heating  lamps  can  be  focused  on  the  resin/fiber  mass  to  affect  partial  cure  of  the  resin 
during  this  lay-down  step.  Once  the  desired  winding  pattern  is  completed,  the  man- 
drel with  its  wound  fiber/resin  composite  outer  surface  is  left  rotating.  Rotation  and 
heat-lamp  curing  continue  until  the  resin  material  is  in  a  rigid  enough  state  that  the 
rotation  can  stop  and  the  cylindrical  part  and  mandrel  can  be  removed  from  the  fila- 
ment winding  machine.  Postcuring  of  the  wound  composite  and  mandrel  can  then  be 
accomplished  by  placing  the  assembly  in  an  oven.  After  final  curing,  the  mandrel  is 
removed  from  the  core  of  the  assembly.  To  facilitate  this,  the  mandrel  form  is  gener- 
ally made  with  a  slight  taper  along  its  length  so  that  the  mandrel  can  easily  be 
slipped  out  of  an  end,  leaving  the  desired  filamentary  composite  cylindrical  "shell." 
The  composite  part  can  then  be  machined  and/or  post-treated  to  the  desired  condition 
or  form. 

Pultrusion  processing.  Pultrusion  is  a  fiber-reinforced  resin  processing  technique 
that  is  readily  adaptable  to  the  continuous  manufacture  of  constant  cross-sectional  lin- 
ear composite  shapes.  Rods,  I  beams,  angles,  channels,  and  hollow  tubes  and  pipes  are 
commonly  produced  by  pultrusion  processing.  Pultrusion  is  a  linear-oriented  process- 
ing method  whereby  yarns  of  reinforcing  fiber  are  continuously  immersed  in  and  im- 
pregnated with  a  catalyzed  fluid  resin.  As  the  term  pultrusion  indicates,  these 
resin-impregnated  continuous-fiber  yarns  are  concurrently  pulled  through  an  elongated 
heated  die  designed  so  that  the  fiber/resin  composite  mass  exiting  the  die  is  sufficiently 
cured  and  retains  the  cross-sectional  shape  of  the  die.  The  apparatus  used  in  the  pul- 
trusion process  is  shown  in  Figure  8.32a.  In  practice,  prescribed  lengths  of  the  formed 


8.5  Fiber-Reinforced  Polymeric  Composites 


321 


FIGURE    8.31 

The  filament  winding 
process:  (a)  apparatus; 
(b)  wind  patterns 


C 


Yarn  spools 
on  a  creel 


Hoop  wind 


C 

Mandrel 


(a) 


Mandrel 


Hoop 


Multiple  helical 


I- 


Hoop  and  helical 
(b) 


Longitudinal 


piece  can  be  cut  using  an  in-line  cutoff  wheel.  Pultrusion  is,  therefore,  adaptable  to 
low-cost,  continuous  production  of  constant  cross-sectional  composite  shapes.  The 
process  of  pultrusion  is  critically  controlled  by  the  resin  system  used  (e.g.,  unsaturated 
polyester,  epoxy,  and  vinyl  ester  resins),  the  temperature  and  temperature  profile  of  the 
heated  die,  and  the  rate  of  pulling  through  the  die. 

In  the  manufacture  of  pultruded  shapes,  such  as  those  shown  in  Figure  8.32b,  al- 
though the  core  cross  section  of  the  composite  is  linear  oriented,  there  is  often  a  need 
to  wrap  the  outer  surface  of  the  composite  with  a  webbing  (nonwoven  or  woven  tape) 
of  fibrous  material.  This  serves  to  consolidate  the  pultruded  shape  and  gives  a  much 
more  durable  outer  surface  to  the  finished  part.  In  this  instance,  thin  veils  of  non- 
woven  or  woven  fabric  tapes  are  fed  into  the  entrance  of  the  die  along  with  the  resin- 
impregnated  continuous-fiber  yarns.  This  assembled  mass  of  fibers  and  resin  proceeds 
to  be  pulled  through  the  die  as  just  described.  The  manufacture  of  hollow  pultruded 
shapes  is  common,  and  a  special  die  is  then  required.  A  shaped  insert  or  "torpedo"  is 
fitted  at  the  die  entrance  and  extends  partway  into  it.  The  fluid  resin-impregnated  fibers 
entering  the  die  are  now  constrained  by  this  center-core  obstruction.  With  the  proper 


322 


8      Plastics 


FIGURE    8.32 

The  pultrusion  process: 
(a)  apparatus;  (b)  cross- 
sectional  designs 


Q^D 


QZD 


Puller 
assembly 


Cured 

composite 

material 


Yarn  spools 
on  a  creel 


(a) 


IT 


^Z^p 


Structural  beams 
shapes 


//////// 

'A 

1 

V/////A 

Beams 


(b) 


Hollow 
pipe  and 
column 


pulling  speed,  die  temperature  profile,  and  catalyzed  resin  formulation,  the  shape  of 
the  insert  is  retained  as  the  desired  hollow  cross  section  of  the  part  exits  the  die. 


Engineering  Design  with  Composite 
Materials 

In  the  development  of  commercial  products,  there  are  many  considerations.  The  par- 
ticular field  of  organic  polymer  engineering  composites  is  no  exception.  It  is  impera- 
tive for  the  engineer  to  have  an  integrated  understanding  of  the  design,  materials 
behavior,  processing,  and  service  performance  behavior  of  composite  materials  in 
order  to  develop  a  successful  product.  This  integrated  approach  is  diagrammed  in  Fig- 
ure 8.33.  Thus  far,  this  section  has  reviewed  some  of  the  materials  and  processing  as- 
pects of  fiber-reinforced  organic  polymer  composites.  The  engineering  design  and  final 
application  aspects  of  composite  materials  are  covered  next. 


8.5  Fiber-Reinforced  Polymeric  Composites 


323 


FIGURE    8.33 

Model  of  technical  base 
for  engineered 
composite  materials 
product  development 


First,  however,  in  order  to  carry  out  an  engineering  design  with  organic  poly- 
mer composites,  the  engineer  must  recognize  and  understand  their  advantages  and 
limitations.  Some  of  the  advantages  and  disadvantages  of  carbon  and  polyaramid 
fiber-reinforced  polymeric  composites  are  as  follows. 

Advantages  of  carbon  fibers. 

1.  High  stiffness-to-weight  and  strength-to-weight  ratios 

2.  High  compressive  strength 

3.  Excellent  fatigue  resistance 

4.  Good  wear  resistance  (self-lubricating)  and  low  friction  coefficient 

5.  Mechanical  vibration  damping  ability  better  than  metals 

6.  Excellent  creep  resistance 

7.  Corrosion  resistance  (when  not  in  contact  with  metals) 

8.  Some  (directional)  electrical  and  thermal  conductivity 

9.  Very  low  (to  slightly  negative)  directional  thermal  expansion  coefficient 

10.  Very  broad  engineering  design  versatility 

11.  Broad  processing  versatility 


324  8      Plastics 


12.  Less  energy  required  to  manufacture  engineering  composite  structures  than  to 
fabricate  with  metals 

Advantages  of  polyaramld  fibers. 

1.  High  stiffness-to-weight  and  strength-to-weight  ratios 

2.  Excellent  fatigue  resistance 

3.  Excellent  corrosion  resistance 

4.  Good  vibration  damping  properties 

5.  Better  impact  resistance  than  carbon  fiber  composites 

6.  Electrically  insulating 

Disadvantages. 

1.  Limited  service  temperature 

2.  Moisture  sensitivity/swelling/distortion 

3.  Anisotropic  properties 

4.  Low  compression  strength  (polyaramid  fiber) 

5.  Bimetallic  corrosion  (carbon  fiber) 

6.  Relatively  high  cost  of  advanced  fibers 

With  these  features  and  limitations  in  mind,  the  design  engineer  can  proceed  to 
create  unique  products.  In  the  composites  field,  it  is  not  appropriate  to  think  only  of 
using  composite  materials  as  a  materials  replacement  for  existing  products.  New  prod- 
ucts that  take  advantage  of  the  unique  properties  of  composite  materials  can  also  be 
conceived.  Many  of  these  new  product  concepts  involve  exploiting  the  remarkable 
specific  strengths  and  specific  elastic  moduli  of  the  "advanced"  fiber- reinforced  com- 
posites. The  design  engineer  can  choose  from  a  multitude  of  reinforcing  fiber  types 
and  fiber  geometry  arrangements,  as  well  as  from  a  variety  of  matrix  materials.  He  or 
she  has  the  freedom  to  mix  in  the  design  specification  two  or  more  diverse  fiber  types, 
as  well  as  the  freedom  to  directionally  place  the  reinforcing  fibers.  All  these  degrees 
of  freedom  of  choice  are  available  so  that  the  desired  final  component  can  be  de- 
signed and  fabricated.  Fiber-reinforced  organic  polymer  engineering  composites  are, 
therefore,  capable  of  being  used  to  create  what  can  be  referred  to  as  integral  design 
engineering  material  structures  (IDEMS).  Through  computer-aided  design  (CAD) 
and  finite-element  stress  analysis  (FEM)  techniques,  new  products  are  developed  in 
computer-model  form.  In  creating  the  actual  fabricated  product,  the  other  facets  of  the 
integrated  materials  system  manufacturing  operation  come  into  play  (see  Figure  8.33). 
Some  specific  areas  in  the  design  of  organic  polymer  composites  are  discussed  next. 

Cutting,  hole  drilling,  and  machining.  Although  composite  parts  and  structures  are 
process  molded  to  the  near-finished  state,  machining,  drilling,  and  trimming  are  often 
required  as  final  steps.  Therefore,  the  assembly  and  the  finishing  of  the  fabricated  part 
are  important  in  the  creation  of  a  final  commercial  product.  There  is  always  the  possi- 
bility of  damaging  the  composite  material  in  these  finishing  post-treatments.  Delamina- 


8.5  Fiber-Reinforced  Polymeric  Composites  325 

tion,  edge  fraying,  matrix  cracking,  or  crazing  leading  to  weak  spots  in  the  composite 
material  structure  are  all  possible.  Great  care  must  be  taken  to  maintain  the  compos- 
ite's structural  integrity  and  appearance. 

Post-treatment  of  fiber-reinforced  composites  involves  different  tooling  and  pro- 
cedures compared  to  what  is  done  for  metal  or  plastics.  The  abrasiveness  of  the  fiber 
and  the  possibility  of  fragmentation  of  the  matrix  resin  are  two  factors  to  consider. 
Composites  are  machined,  cut,  and  trimmed  more  easily  using  processes  similar  to 
grinding  or  abrasive  cutting  rather  than  conventional  metal-cutting  techniques.  Also, 
the  method  used  is  dictated  by  the  type  of  fiber  reinforcement.  Glass  fiber,  carbon 
fiber,  and  especially  polyaramid  fiber  composites  all  require  their  own  procedures. 
For  example,  the  cutting  of  polyaramid-fiber-reinforced  composites  is  difficult  be- 
cause the  fiber  is  so  tough  and  does  not  cleave  or  cut  in  a  brittle,  fracture  mode.  Pol- 
yaramid fibers  undergo  a  process  called  fibrillation  when  "damaged"  by  the  drilling, 
cutting,  or  machining  tool.  Fuzzy  edge  cuts  or  fiber-filled  drill  holes  are  produced 
when  conventional  machining  and  drilling  tools  are  used.  For  polyaramid  and  for 
other  fiber-reinforced  composite  materials  for  that  matter,  water-jet  cutting,  laser  cut- 
ting, and  diamond  wire  cutting  are  often  used  to  achieve  an  acceptable  edge  profile 
to  the  final  machine-finished  parts.  For  carbon-fiber-reinforced  composites,  the  ther- 
mal effects  due  to  laser  cutting,  machining,  and  drilling  can  be  a  deterrent  because 
the  carbon  fibers  are  thermally  conductive.  A  weakened,  charred,  heat-damaged  zone 
may  surround  the  laser-cut  edge.  In  summary,  great  care  must  be  taken  in  the  finish- 
ing post-treatments  of  fiber-reinforced  composite  materials. 

Adhesive  and  mechanical  joining.  Adhesives  are  the  principal  means  of  joining  com- 
posite materials  to  themselves  and  other  materials  of  construction  (metals,  plastics, 
wood).  The  reasons  for  this  are  numerous.  Most  importantly,  adhesive  bonds  are 
uniquely  capable  of  distributing  stress  and  can  easily  be  joined  into  contoured  shapes. 
In  mechanical  joining,  hole  drilling  is  required,  which  can  lead  to  delamination  of  the 
composite  and  a  stress  concentration  at  the  point  of  joining.  The  transfer  of  load  from 
one  material  to  another  without  creating  large  stress  concentrations  is  the  ultimate  goal 
of  materials  joining.  This  can  be  achieved  better  by  adhesive  joining.  Adhesives  can 
often  be  incorporated  into  the  structural  laminar  shape  being  fabricated  as  a  one-step 
manufacturing  process.  Metal  strips,  layers,  and/or  fittings  can  easily  be  adhesively 
"molded"  in  the  manufactured  structure  during  the  composite  processing  stage  (e.g., 
wet  lay-up,  filament  winding,  RIM,  RRIM,  and  so  on).  Adhesive  joining  techniques 
lend  themselves  to  the  creation  of  integrally  designed  structures  as  described  previ- 
ously. The  various  adhesive  joint  designs  (lap  shear,  butt  tensile,  scarf  joints,  and  so 
on)  were  discussed  in  Chapter  4. 

Structural  adhesives  are  available  in  various  forms  and  types.  Most  common  are 
the  two-package  epoxy  resins.  These  formulated  products  are  very  similar  to  the  epoxy 
matrix  resins  used  to  create  the  fiber-reinforced  composite  materials  themselves.  Usu- 
ally, these  two-package  products  consist  of  part  A,  the  epoxy  resin  prepolymer,  and 
part  B,  the  curative  (such  as  a  primary  amine  or  a  polyamide/amine).  Fillers,  thicken- 
ers, reactive  diluents,  tackifiers,  and  other  processing  aids  such  as  silicone  compounds 
to  improve  the  moisture  durability  of  the  adhesive  are  added  to  the  final  formulation. 


326  8      Plastics 


These  two-part  adhesives  are  mixed  just  before  being  applied  to  the  surfaces  of  the 
parts  to  be  joined.  The  assembly  is  then  placed  in  a  compression  mold,  platen  press,  or 
vacuum  bagging  arrangement,  where  heat  may  be  applied  to  consolidate  the  layers 
being  joined  and  cure  the  adhesive.  There  are  also  some  one-package  paste  adhesives 
that  are  formulated  with  a  latent  curative;  the  curative  reacts  only  at  high  temperature. 

Another  useful  form  of  adhesive  is  the  film  adhesive.  Film  adhesives  are  used  ex- 
tensively in  the  aerospace  industry.  Here,  adhesives  exist  in  the  form  of  sheets.  These 
sheets  are  malleable,  are  drapable,  and  can  be  cut  using  shears  to  the  desired  size  and 
shape.  These  films  are  then  placed  between  the  surfaces  to  be  joined  and  are  cured 
under  consolidation  pressure  and  elevated  temperature.  Like  the  one-package  adhe- 
sives, these  adhesives  are  formulated  with  a  high-temperature-reacting  latent  curative. 
Film  adhesives,  like  the  fiber-reinforced  epoxy  prepregs  described  earlier,  must  be 
stored  at  low  temperature  and  kept  frozen  until  ready  to  use. 

Also  used  in  bonding  composite  materials  are  the  acrylic  adhesives.  Acrylic  ad- 
hesives having  different  flexibilities  are  available.  They  cure  at  room  temperature  by  a 
free-radical  polymerization  reaction.  One  feature  of  acrylic  adhesives  is  that  cure  can 
be  achieved  by  first  coating  the  free-radical  catalyst  on  the  surfaces  to  be  bonded.  This 
"catalyst-primed"  surface  can  then  be  stored  until  it  is  ready  for  bonding.  An  uncat- 
alyzed  acrylic  adhesive  is  then  coated  onto  the  catalyst-primed  surface.  The  surfaces 
to  be  joined  are  then  mated  under  contact  pressure  and  allowed  to  cure,  undisturbed, 
at  room  temperature.  Acrylic  adhesives  can  produce  bonds  that  are  very  oil  resistant. 

Finally,  it  is  important  that  the  surfaces  to  be  joined  be  clean  and  free  of  oils, 
greases,  and  loose  surface  material  layers.  This  is  especially  necessary  when  joining 
composite  materials  to  metals.  Vapor  degreasing,  followed  by  a  chemically  alkaline 
cleaning  bath,  is  normally  used  for  surface  treating  metals  prior  to  adhesive  bonding. 

Sandwich-panel  construction.  Structural  sandwich-panel  construction  consists  of 
face  sheets  made  up  of  fiber-reinforced  laminar  composite  material  (or  metal  sheet) 
adhesively  bonded  to  both  sides  of  a  core  material.  This  concept  is  illustrated  in  Fig- 
ure 8.34.  The  principle  behind  sandwich  construction  is  that  the  core  material  spaces 
the  facings  away  from  the  symmetric  center  of  the  panel.  Therefore,  in  flexure,  the 
faces  or  outer  skins  of  the  panel  are  in  tension  or  compression.  This  construction  leads 
to  the  reinforcement  in  the  faces,  which  resists  the  bending  of  the  panel.  The  columnar 
strength  of  the  honeycomb  core  material  then  provides  the  shear  and  compression 
strength  of  this  unique  panel  structure.  Above  all,  the  adhesive  must  be  strong  and 
have  a  high  enough  shear  and  peel  strength  to  withstand  these  shear  stresses. 

Sandwich  construction  leads  to  the  use  of  panels  that  give  the  highest  stiffness-to- 
weight  ratio  of  any  material  design.  Sandwich-panel  construction  is  used  extensively 
in  aircraft  and  aerospace  applications,  where  the  core  materials  are  generally  honey- 
combed in  geometric  shape.  Honeycomb  cores  can  be  made  of  thin  metal  (aluminum 
or  titanium)  or  of  fiber-reinforced  resin  sheet  (e.g.,  thin  sheet  of  resin-impregnated 
glass,  carbon,  or  polyaramid  mat).  The  manufacture  of  honeycomb  core  by  the  expan- 
sion process  is  shown  in  Figure  8.35.  Manufacturing  honeycomb  core  involves  coating 
discrete  strips  of  adhesive  onto  sheets  of  core  material.  The  specially  coated  core  ma- 
terial is  then  cured  under  compression  to  form  a  "log"  or  block  of  core  material.  The 


8.5  Fiber-Reinforced  Polymeric  Composites 


327 


FIGURE    8.34 

Structural  sandwich 
panel  construction 
(Courtesy  Strong,  A.  B. 
Fundamentals  of 
Composites 
Manufacturing: 
Materials,  Methods, 
and  Applications. 
Dearborn,  Michigan: 
Society  of 
Manufacturing 
Engineers,  1989) 


Face  sheet 


Honeycomb  (Metal,  composite,  or  paper) 


Film  adhesive 


Face  sheet 


log  must  then  be  cut  to  the  desired  core  height  and  subsequently  expanded  to  form  the 
final  core  material.  In  some  instances,  the  core  material  is  dipped  into  a  resin  solution 
so  that  the  core  structure  can  be  consolidated  or  stiffened.  Another  method  of  making 
honeycomb  is  the  direct  corrugation  process.  In  some  less  demanding  stiffness  and 
compression  applications,  a  rigid  foam  core  material  can  be  used.  Rigid  foam  and 


FIGURE    8.35 

Manufacture  of 
honeycomb  core  by  the 
expansion  process 


1 .  Adhesive 
strips  are 
coated  onto 
web. 

Adhesive 


2.  Plies  from  step  1  are  laid 
to  form  a  block. 


3.  Block  is  cured  under 
heat  and  compression. 


h 

II  II 
II 

u  u 

h 

II 
II 

u 

1       2      3      4      5      6. 

Plies-adhesive 
coat  index 

4.  Expansion  leads  to 

formation  of  honeycomb  cone. 


328 


8      Plastics 


Kraft-paper-based  honeycomb  core  panels  are  often  used  in  truck  cargo  bed  panels  and 
in  door  panels. 

Painting  and  coating.  Standard  coating  methods  can  be  used  for  painting  or  coating 
fiber-reinforced  composite  structures.  In  all  cases,  the  surface  of  the  composite  must 
be  thoroughly  prepared  before  the  final  coating  is  applied.  Surface  cleaning,  sanding, 
abrading,  filling  in  surface  grooves/blemishes,  and  a  solvent  wipe  must  be  carried  out 
before  the  paint  sealer  and  final  paint  finish  are  applied.  Paint  sealers  and  the  final 
paint  coating  must  be  dried/cured  at  temperatures  below  the  cure  temperature  of  the 
composite  part.  Drying  with  infrared  heaters  can  be  troublesome  as  the  heat  location 
and  temperature  cannot  be  properly  controlled  using  this  technique.  Epoxy  and 
polyurethane-based  surface  coatings  are  especially  useful  in  the  painting  of  composite 
structures. 


REFERENCES 


Kaverman,  R.  D.  "Reinforced  Plastics  and  Compos- 
ites." In  Michael  L.  Berins,  ed.,  SPI  Plastics  Engi- 
neering Handbook.  New  York:  Van  Nostrand 
Reinhold,  1991. 

Mayer,  Rayner  M.  Design  with  Reinforced  Plastics. 
Design  Council,  K128  Haymarket,  London  SWIY 
450.  Bournemouth,  England:  Bourne  Press  Ltd., 
1995. 


Schwartz,  Mel  M.,  ed.  Composite  Materials  Hand- 
book, 2nd  ed.  New  York:  McGraw-Hill,  1992. 

Strong,  A.  B.  Fundamentals  of  Composites  Manu- 
facturing: Materials,  Methods,  and  Applications. 
Dearborn,  Michigan:  Society  of  Manufacturing  En- 
gineers, 1989. 


Review  Questions 


,v, 


1.  What  are  plastics  and  why  are  they  called  poly- 
mers'? 

2.  What  is  a  monomer"? 

3.  Are  all  polymers  artificial?  Give  examples. 

4.  Why  is  the  strength  of  polymers  lower  than  that 
of  metals? 

5.  Why  is  the  electrical  conductivity  of  polymers 
lower  than  that  of  metals? 

6.  When  did  polymers  start  to  gain  widespread  ap- 
plication and  why? 

7.  How  are  polymers  classified  based  on  their 
temperature  characteristics? 


8.  What  is  meant  by  chemical  families  of  poly- 
mers? Give  examples. 

9.  What  are  the  main  characteristics  of  a  thermo- 
plastic polymer? 

10.  Does  a  thermoplastic  polymer  have  a  fixed 
melting  temperature?  Why? 

11.  What  is  meant  by  shaping  memoryl 

12.  What  are  the  main  characteristics  of  a  ther- 
mosetting polymer? 

13.  How  do  molecules  of  a  thermosetting  polymer 
differ  from  those  of  a  thermoplastic  polymer? 


Chapter  8  Review  Questions 


329 


14.  Compare  the  properties  of  plastics  with  those  of 
metals.  How  do  the  differences  affect  the  de- 
sign of  plastic  products? 

15.  How  can  we  have  different  polymers  starting 
from  the  same  monomer? 

16.  List  four  polymers  that  belong  to  the  ethenic 
group.  Discuss  their  properties  and  applica- 
tions. 

17.  What  are  the  main  applications  of  polyacetals? 

18.  What  is  cellophane  and  how  is  it  produced? 

19.  What  is  the  major  disadvantage  of  cellulose  ni- 
trate? 

20.  What  are  the  major  applications  for  cellulose 
acetate? 

21.  What  is  the  chief  limitation  of  nylons? 

22.  What  are  the  major  characteristics  of  pheno- 
lics? 

23.  How  are  polyimides  manufactured? 

24.  List  the  common  applications  for  epoxies. 

25.  Discuss  the  properties  of  polyurethanes  and  list 
some  of  their  applications. 

26.  What  property  characterizes  silicones?  Suggest 
suitable  applications  to  make  use  of  that  prop- 
erty. 

27.  Explain  how  natural  rubber  is  processed. 

28.  Why  are  additives  compounded  with  polymers? 

29.  List  some  fillers.  Why  are  they  added  to  poly- 
mers? 

30.  What  happens  when  too  much  filler  is  added? 

31.  How  does  the  addition  of  plasticizers  affect  the 
properties  of  a  polymer? 

32.  List  some  of  the  lubricants  used  when  process- 
ing polymers. 

33.  What  are  the  mechanisms  for  coloring  poly- 
mers? 

34.  Are  all  polymers  cast  in  the  same  manner? 

35.  What  are  the  design  features  of  parts  produced 
by  blow  molding? 


36.  Using  sketches,  explain  the  injection  molding 
process. 

37.  What  is  the  chief  limitation  of  injection  mold- 
ing? 

38.  What  kinds  of  polymers  are  usually  processed 
by  compression  molding? 

39.  List  some  advantages  of  the  compression  mold- 
ing process. 

40.  What  is  the  main  difference  between  compres- 
sion molding  and  transfer  molding? 

41.  Explain  briefly  the  operating  principles  of  rota- 
tional molding. 

42.  List  examples  of  plastic  products  that  are  man- 
ufactured by  extrusion. 

43.  What  is  the  coextrusion  process?  Why  is  it  used 
in  industry? 

44.  What  are  the  design  features  of  parts  produced 
by  thermoforming?  Give  examples. 

45.  What  are  the  products  of  the  calendering 
process? 

46.  What  is  the  major  problem  experienced  when 
machining  plastics? 

47.  Using  sketches,  explain  the  process  of  hot-plate 
joining. 

48.  Describe  thermal  staking. 

49.  Explain  how  ultrasonics  are  employed  in  weld- 
ing and  assembling  plastic  parts. 

50.  Do  all  plastics  render  themselves  suitable  for 
ultrasonic  welding?  Explain. 

51.  What  are  the  basic  components  of  ultrasonic 
welding  equipment? 

52.  Using  sketches,  show  some  designs  of  ultra- 
sonic-welded joints.  List  the  characteristics  of 
each. 

53.  Explain  the  sequence  of  operations  involved  in 
open-mold  processing  of  reinforced  polymers. 

54.  What  are  the  similarities  and  differences  be- 
tween extrusion  and  pultrusion  of  polymers? 

55.  What  are  the  design  features  of  parts  manufac- 
tured by  filament  winding? 


330 


8      Plastics 


56.  Explain  briefly  the  nature  of  FRP  composites. 

57.  How  can  you  predict  the  properties  of  a  com- 
posite? Provide  a  quantitative  equation. 

58.  List  some  of  the  fibers  used  as  inforcement  in 
FRP  composites. 

59.  Briefly  discuss  the  various  matrix  resins  for 
FRP  composites  indicating  their  advantages, 
disadvantages,  and  limitations. 


60.  Why  is  vacuum  bagging  used  in  the  modified 
version  of  the  wet  lay-up  method? 

61.  What  should  we  be  careful  about  when  using 
fiber  resin  prepregs? 

62.  What  are  the  advantages  of  sandwich  panels? 


Design  Pxojects__ 


The  current  products  of  a  company  involve  dif- 
ferent fruit  preserves  in  tin  cans,  each  containing 
8  ounces  (about  250  g).  The  company  uses 
250,000  tin  cans  annually,  and  each  costs  13 
cents.  Because  their  machines  are  almost  obso- 
lete and  the  cost  of  tin  is  rising  every  year,  the 
company  is  considering  replacing  the  tin  cans 
with  plastic  containers.  Design  plastic  containers 
to  serve  this  goal,  taking  into  account  the  plastic- 
processing  method  to  be  used.  Also,  make  a  fea- 
sibility study  for  the  project. 

Design  a  plastic  cup  that  has  a  capacity  of  8 
ounces  (about  250  g)  of  water.  Assume  the  an- 
nual production  volume  is  20,000  pieces. 

Design  a  high-quality  plastic  pitcher  that  has  a 
capacity  of  32  ounces  (about  1  kg)  of  liquid.  As- 


I 

sume  the  annual  production  volume  is  15,000 
pieces. 

Design  a  wheel  for  a  bicycle  so  that  it  can  be 
produced  by  injection  molding  instead  of  sheet 
metal  forming.  The  diameter  is  24  inches  (600 
mm),  and  a  load  of  100  pounds  (about  45  kg)  is 
applied,  through  the  axle,  at  its  center.  Assume 
the  annual  production  volume  is  100,000  wheels. 

A  trash  container  that  has  a  capacity  of  1  cubic 
foot  (0.027  m3)  is  made  of  sheet  metal  and  can 
withstand  an  axial  compressive  load  of  110 
pounds  (50  kg).  Redesign  it  so  that  it  can  be 
made  of  plastic.  Assume  the  annual  production 
volume  is  20,000  pieces. 


Chapter  9 


yslcs  of 


Metal  Cutting 


INTRODUCTION 

Metal  cutting  can  be  defined  as  a  process  during  which  the  shape  and  dimen- 
sions of  a  workpiece  are  changed  by  removing  some  of  its  material  in  the  form 
of  chips.  The  chips  are  separated  from  the  workpiece  by  means  of  a  cutting 
tool  that  possesses  a  very  high  hardness  compared  with  that  of  the  workpiece, 
as  well  as  certain  geometrical  characteristics  that  depend  upon  the  conditions 
of  the  cutting  operation.  Among  all  of  the  manufacturing  methods,  metal  cut- 
ting, commonly  called  machining,  is  perhaps  the  most  important.  Forgings  and 
castings  are  subjected  to  subsequent  machining  operations  to  acquire  the  pre- 
cise dimensions  and  surface  finish  required.  Also,  products  can  sometimes  be 
manufactured  by  machining  stock  materials  like  bars,  plates,  or  structural  sec- 
tions. 

Machining  comprises  a  group  of  operations  that  involve  seven  basic  chip- 
producing  processes:  shaping,  turning,  milling,  drilling,  sawing,  broaching,  and 
grinding.  Although  one  or  more  of  these  metal-removal  processes  are  performed 
at  some  stage  in  the  manufacture  of  the  vast  majority  of  industrial  products,  the 
basis  for  all  these  processes  (i.e.,  the  mechanics  of  metal  cutting)  is  yet  not  fully 
or  perfectly  understood.  This  is  certainly  not  due  to  the  lack  of  research  but 
rather  is  caused  by  the  extreme  complexity  of  the  problem.  A  wide  variety  of  fac- 
tors contribute  to  this  complexity,  including  the  large  plastic  strains  and  high 
strain  rates  involved,  the  heat  generated  and  high  rise  in  temperature  during  ma- 
chining, and,  finally,  the  effect  of  variations  in  tool  geometry  and  tool  material.  It 
seems,  therefore,  realistic  to  try  to  simplify  the  cutting  operation  by  eliminating 


331 


332 


9      Physics  of  Metal  Cutting 


FIGURE    9.1 

Two-dimensional  cutting 
using  a  prismatic, 
wedge-shaped  tool 


Workpiece 


as  many  of  the  independent  variables  as  possible  and  making  appropriately  im- 
plicit assumptions  if  an  insight  into  this  complicated  process  is  to  be  gained.  In 
fact,  we  are  going  to  take  this  approach  in  discussing  the  cutting  tools  and  the 
mechanics  of  chip  formation.  We  are  going  to  consider  two-dimensional  cutting, 
in  which  a  prismatic,  wedge-shaped  tool  with  a  straight  cutting  edge  is  employed, 
as  shown  in  Figure  9.1,  and  the  direction  of  motion  of  the  tool  (relative  to  the 
workpiece)  is  perpendicular  to  its  straight  cutting  edge.  In  reality,  such  condi- 
tions resemble  the  case  of  machining  a  plate  or  the  edge  of  a  thin  tube  and  are 
referred  to  as  orthogonal  cutting. 


UTTING  ANGLES 


Figure  9.2  clearly  illustrates  that  the  lower  surface  of  the  tool,  called  the  flank,  makes 
an  angle  \j/  with  the  newly  machined  surface  of  the  workpiece.  This  clearance  angle  is 
essential  for  the  elimination  of  friction  between  the  flank  and  the  newly  machined  sur- 
face. As  can  also  be  seen  in  Figure  9.2,  there  is  an  angle  a  between  the  upper  surface, 
or  face,  of  the  tool  along  which  chips  flow  and  the  plane  perpendicular  to  the  machined 


FIGURE    9.2 

Tool  angles  in  two- 
dimensional  cutting 


Tool  angle 


Clearance 
angle 


9.1  Cutting  Angles 


333 


surface  of  the  workpiece.  It  is  easy  to  realize  that  the  angle  a  indirectly  specifies  the 
slope  of  the  tool  face.  This  angle  is  known  as  the  rake  angle  and  is  necessary  for  shov- 
eling the  chips  formed  during  machining  operations.  The  resistance  to  the  flow  of  the 
removed  chips  depends  mainly  upon  the  value  of  the  rake  angle.  As  a  consequence,  the 
quality  of  the  machined  surface  also  depends  on  the  value  of  the  rake  angle.  In  addi- 
tion to  these  two  angles,  there  is  the  tool  angle  (or  wedge  angle),  which  is  the  angle 
confined  between  the  face  and  the  flank  of  the  tool.  Note  that  the  algebraic  sum  of  the 
rake,  tool,  and  clearance  angles  is  always  equal  to  90°.  Therefore,  it  is  sufficient  to  de- 
fine only  two  of  these  three  angles.  In  metal-cutting  practice,  the  rake  and  clearance 
angles  are  the  ones  that  are  defined. 

As  you  may  expect,  the  recommended  values  for  the  rake  and  clearance  angles  are 
dependent  upon  the  nature  of  the  metal-cutting  operation  and  the  material  of  the  work- 
piece  to  be  machined.  The  choice  of  proper  values  for  these  two  angles  results  in  the 
following  gains: 

1.  Improved  quality  of  the  machined  surface 

2.  A  decrease  in  the  energy  consumed  during  the  machining  operation  (most  of 
which  is  converted  into  heat) 

3.  Longer  tool  life  as  a  result  of  a  decrease  in  the  rate  of  tool  wear  because  the 
elapsed  heat  is  reduced  to  minimum 

Let  us  now  consider  how  the  mechanical  properties  of  the  workpiece  material  af- 
fect the  optimum  value  of  the  rake  and  clearance  angles.  Generally,  soft,  ductile  met- 
als require  tools  with  larger  positive  rake  angles  to  allow  easy  flow  of  the  removed 
chips  on  the  tool  face,  as  shown  in  Figure  9.3.  In  addition,  the  higher  the  ductility  of 
the  workpiece  material,  the  larger  the  tool  clearance  angle  that  is  needed  in  order  to  re- 
duce the  part  of  the  tool  that  will  sink  into  the  workpiece  (i.e.,  reduce  the  area  of  con- 
tact between  the  tool  flank  and  the  machined  workpiece  surface).  On  the  other  hand, 
hard,  brittle  materials  require  tools  with  smaller  or  even  negative  rake  angles  in  order 
to  increase  the  section  of  the  tool  subjected  to  the  loading,  thus  enabling  the  tool  to 
withstand  the  high  cutting  forces  that  result.  Figure  9.4  illustrates  tools  having  zero  and 
negative  rake  angles  required  when  machining  hard,  brittle  alloys.  In  this  case,  the 
clearance  angle  is  usually  taken  as  smaller  than  that  recommended  when  machining 
soft,  ductile  materials. 


FIGURE    9.3 

Positive  rake  angle 
required  when 
machining  soft,  ductile 
metals 


Positive 
rake  angle 


Workpiece 


334 


9      Physics  of  Metal  Cutting 


FIGURE    9.4 

Zero  and  negative  rake 
angles  required  when 
machining  hard,  brittle 
materials 


Zero 
rake  angle 


Negative 
rake  angle. 


Workpiece 


Tool 


Workpiece 


7 


9.2  CHIP  FORMATION 


Mechanics  of  Chip  Formation 

There  was  an  early  attempt  by  Reuleaux  at  the  beginning  of  the  twentieth  century  to 
explain  the  mechanics  of  chip  formation.  He  established  a  theory  that  gained  popular- 
ity for  many  years;  it  was  based  on  assuming  that  a  crack  would  be  initiated  ahead  of 
the  cutting  edge  and  would  propagate  in  a  fashion  similar  to  that  of  the  splitting  of 
wood  fibers,  as  shown  in  Figure  9.5.  Thanks  to  modern  research  that  employed  high- 
speed photography  and  quick  stopping  devices  capable  of  freezing  the  cutting  action, 
it  was  possible  to  gain  a  deeper  insight  into  the  process  of  chip  formation.  As  a  result, 
Reuleaux's  theory  collapsed  and  proved  to  be  a  misconception;  it  has  been  found  that 
the  operation  of  chip  formation  basically  involves  shearing  of  the  workpiece  material. 
Let  us  now  see,  step  by  step,  how  that  operation  takes  place. 

The  stages  involved  in  chip  removal  are  shown  in  Figure  9.6.  When  the  tool  is  set 
at  a  certain  depth  of  cut  (see  Figure  9.6a)  and  is  then  pushed  against  the  workpiece,  the 
cutting  edge  of  the  tool  and  the  face  start  to  penetrate  the  workpiece  material.  The  sur- 
face layer  of  the  material  is  compressed;  then  pressure  builds  up  and  eventually  ex- 
ceeds the  elastic  limit  of  the  material.  As  a  result  of  the  intense  shear  stress  along  the 
plane  N-N,  called  the  shear  plane,  plastic  deformation  takes  place,  and  the  material  of 
the  surface  layer  has  no  option  but  to  flow  along  the  face  of  the  tool  without  being  sep- 
arated from  the  rest  of  the  workpiece  (see  Figure  9.6b).  With  further  pushing  of  the 
tool,  the  ultimate  tensile  strength  is  exceeded,  and  a  little  piece  of  material  (a  chip)  is 


FIGURE    9.5 

Reuleaux's 
misconception  of  the 
mechanics  of  chip 
removal 


Workpiece 


9.2  Chip  Formation 


335 


FIGURE    9.6 

Stages  in  chip  removal: 
(a)  tool  set  at  a  certain 
depth  of  cut  set;  (b) 
workpiece  penetration; 
(c)  chip  separation 


Workpiece 


Tool 


Chip 
separated 


A  new  chip 

being 
generated 


(b) 


(0 


separated  from  the  workpiece  by  slipping  along  the  shear  plane  (see  Figure  9.6c).  This 
sequence  is  repeated  as  long  as  the  tool  continues  to  be  pushed  against  the  workpiece, 
and  the  second,  third,  and  subsequent  chips  are  accordingly  separated. 

Types  of  Chips 

The  type  of  chip  produced  during  metal  cutting  depends  upon  the  following  factors: 

1.  The  mechanical  properties  (mainly  ductility)  of  the  material  being  machined 

2.  The  geometry  of  the  cutting  tool 

3.  The  cutting  conditions  used  (e.g.,  cutting  speed)  and  the  cross-sectional  area  of 
the  chip 

Based  on  these  factors,  the  generated  chips  may  take  one  of  the  forms  shown  in  Fig- 
ure 9.7.  Following  is  a  discussion  of  each  type  of  chip. 

Continuous  chip.  When  machining  soft,  ductile  metals  such  as  low-carbon  steel,  cop- 
per, and  aluminum  at  the  recommended  cutting  speeds  (which  are  high),  plastic  flow 
predominates  over  shearing  (i.e.,  plastic  flow  continues,  and  shearing  of  the  chip  never 
takes  place).  Consequently,  the  chip  takes  the  form  of  a  continuous,  twisted  ribbon  (see 
Figure  9.7a).  Because  the  energy  consumed  in  plastically  deforming  the  metal  is  even- 
tually converted  into  heat,  coolants  and  lubricants  must  be  used  to  remove  the  gener- 
ated heat  and  to  reduce  friction  between  the  tool  face  and  the  hot,  soft  chip. 

Discontinuous  chips.  When  machining  hard,  brittle  materials  such  as  cast  iron  or 
bronze,  brittle  failure  takes  place  along  the  shear  plane  before  any  tangible  plastic  flow 
occurs.  Consequently,  the  chips  take  the  form  of  discontinuous  segments  with  irregu- 
lar shape  (see  Figure  9.7b).  As  no  plastic  deformation  is  involved,  there  is  no  energy 
to  be  converted  into  heat.  Also,  the  period  of  time  during  which  a  chip  remains  in  con- 


FIGURE    9.7 

Types  of  machining 
chips:  (a)  continuous, 
twisted  ribbon;  (b) 
discontinuous,  irregular 
segments;  (c)  sheared, 
short  ribbons 


^Sfflto% 


(a) 


(b) 


(c) 


336 


Physics  of  Metal  Cutting 


tact  with  the  face  of  the  tool  is  short,  and.  therefore,  the  heat  generated  due  to  friction 
is  very  small.  As  a  result,  the  tool  does  not  become  hot,  and  lubricants  and  coolants  are 
not  required. 

Sheared  chips.  When  machining  semiductile  materials  with  heavy  cuts  and  at  rela- 
tively low  cutting  speeds,  the  resulting  sheared  chips  have  a  shape  that  is  midway  be- 
tween the  segmented  and  the  continuous  chips  (see  Figure  9.7c).  They  are  usually 
short,  twisted  ribbons  that  break  every  now  and  then. 

The  Problem  of  the  Built-Up  Edge 

When  machining  highly  plastic,  tough  metals  at  high  cutting  speeds,  the  amount  of 
heat  generated  as  a  result  of  plastic  deformation  and  friction  between  the  chip  and  the 
tool  is  large  and  results  in  the  formation  of  a  built-up  edge,  as  shown  in  Figure  9.8.  The 
combination  of  the  resulting  elevated  temperature  with  the  high  pressure  at  the  tool 
face  causes  localized  welding  of  some  of  the  chip  material  to  the  tool  face  (see  Figure 
9.8a).  The  welded  material  (chip  segment)  becomes  an  integral  part  of  the  cutting  tool, 
thus  changing  the  values  of  the  cutting  angles.  This  certainly  increases  friction,  lead- 
ing to  the  buildup  of  layer  upon  layer  of  chip  material.  This  newly  formed  false  cut- 
ting edge  (see  Figure  9.8b)  is  referred  to  as  the  built-up  edge.  The  cutting  forces  also 
increase,  the  built-up  edge  breaks  down,  and  the  fractured  edges  adhere  to  the  ma- 
chined surface  (see  Figure  9.8c).  The  harmful  effects  of  the  built-up  edge  are  increased 
tool  wear  and  a  very  poorly  machined  workpiece.  The  manufacturing  engineer  must 
choose  the  proper  cutting  conditions  to  avoid  the  formation  of  a  continuous  chip  with 
a  built-up  edge. 

The  Cutting  Ratio 

As  can  be  seen  in  Figure  9.9.  during  a  cutting  operation,  the  workpiece  material  just 
ahead  of  the  tool  is  subjected  to  compression,  and,  therefore,  the  chip  thickness  be- 
comes greater  than  the  depth  of  cut.  The  ratio  of  t0/t  is  called  the  cutting  ratio  (rc)  and 


FIGURE    9.8 

Stages  in  the  formation 
of  the  built-up  edge:  (a) 
localized  welding;  (b) 
false  cutting  edge;  (c) 
flawed  surface 


Built-up 
edge 


(a) 


(b) 


Broken  chips 

sticking  to  the 

newly  machined 

surface 


(c) 


9.2  Chip  Formation 


337 


FIGURE    9.9 

Geometry  of  a  chip  with 
respect  to  depth  of  cut 


can  be  obtained  as  follows: 


sin  (|) 


r  _*o  _        ts  sin  $ 

t      ts  cos  (())  -  a)      cos  (§  -  a) 


(9.1) 


By  employing  trigonometry  and  carrying  out  simple  mathematical  manipulation,  we 
can  obtain  the  following  equation: 


tan  <b  = 


rc  cos  a 
1  -  rc  sin  a 


(9.2) 


Equation  9.2  is  employed  in  obtaining  the  value  of  the  shear  angle  §  when  the  rake 
angle  a,  the  depth  of  cut,  and  the  final  thickness  of  the  chips  are  known.  In  experi- 
mental work,  the  chip  thickness  is  either  measured  directly  with  the  help  of  a  ball- 
ended  micrometer  or  obtained  from  the  weight  of  a  known  length  of  chip  (of  course, 
the  density  and  the  width  of  the  chip  must  also  be  known). 

Let  us  now  study  the  relationship  between  velocities.  Considering  the  constancy 
of  mass  and  assuming  the  width  of  the  chip  to  remain  constant,  it  is  easy  to  see  that 


or 


V  x  tn  =  V,  x  t 


V      t       c 


In  other  words, 


Vr  =  Vrr  = 


V  sin  (j) 
cos  (())  -  a) 


(9.3) 


We  can  now  draw  the  velocity  triangle  because  we  know  the  magnitudes  and  directions 
of  two  velocities,  V  and  Vc.  The  shear  velocity,  Vv,  which  is  the  velocity  with  which  the 
metal  slides  along  the  shear  plane,  can  then  be  determined.  Based  on  the  velocity  tri- 
angle shown  in  Figure  9.10  and  applying  the  sine  rule,  the  following  can  be  stated: 


V 


V. 


V, 


sin  (90  -  (j»  +  a)      sin  (90  -  a)      sin  ty 


338 


9      Physics  of  Metal  Cutting 


FIGURE    9.10 

Velocity  triangle  and 
kinematics  of  the  chip- 
removal  process 


(90  -  <t>  +  a) 


(0  -a) 


(90  -  a) 


This  equation  can  also  take  the  form 

V  Vs    _    vc 

cos  ((})  -  a)      cos  a      sin  <j) 

Therefore, 

cos  a 


V=V 


cos  ((()  -  a) 


(9.4) 


Shear  Strain  During  Chip  Formation 

The  value  of  the  shear  strain  is  an  indication  of  the  amount  of  deformation  that  the 
metal  undergoes  during  the  process  of  chip  formation.  As  can  be  seen  in  Figure  9.11, 
the  parallelogram  abda'  will  take  the  shape  abed'  due  to  shearing.  The  shear  strain  can 
be  expressed  as  follows: 


a'n      d'n  .,         . 

y  = h =  cot  (J)  +  tan  (<|)  -  a) 

an       an 


(9.5) 


The  shear  strain  rate  can  be  obtained  from  Equation  9.5  as  follows: 


a'n        1        d'n        1 

y  = x  —  + x  — 

an       At       an       At 


a'd'        1 

x  — 

an       At 


FIGURE    9.11 

Shear  strain  during  chip 
formation 


Rake  angle 


Shear 

strain, 

7 


Final  shape  of  material 
,         just  after  deformation 
^   y         (broken  line 


Original  shape  of 
material  just  before 
machining  (hatched) 


9.3  Cutting  Forces 


339 


But, 


a'd' 


At 
Therefore, 


=  K 


Y  = 


V. 


an 


where  an  is  the  thickness  of  the  shear  zone.  Experimental  results  have  indicated  that 
the  thickness  of  the  shear  zone  is  very  small.  Consequently,  it  can  easily  be  concluded 
that  the  process  of  chip  formation  takes  place  at  an  extremely  high  strain  rate.  This 
finding  is  very  important,  especially  for  strain-rate-sensitive  materials,  where  the 
strength  and  ductility  of  the  material  are  markedly  affected. 


UTTING  FORCES 


Theory  of  Ernst  and  Merchant 

In  order  to  simplify  the  problem,  let  us  consider  the  two-dimensional,  idealized  cut- 
ting model  of  continuous  chip  formation.  In  this  case,  all  the  forces  lie  in  the  same 
plane  and,  therefore,  form  a  coplanar  system  of  forces.  Walter  Ernst  and  Eugene  M. 
Merchant,  both  eminent  American  manufacturing  scientists,  based  their  analysis  of 
this  system  of  forces  on  the  assumption  that  a  chip  acts  as  a  rigid  body  in  equilib- 
rium under  the  forces  acting  across  the  chip-tool  interface  and  the  shear  plane.  As 
Figure  9.12  shows,  the  cutting  edge  exerts  a  certain  force  upon  the  workpiece.  The 
magnitude  of  that  force  is  dependent  upon  many  factors,  such  as  the  workpiece  ma- 
terial, the  conditions  of  cutting,  and  the  values  of  the  cutting  angles. 


FIGURE    9.12 

Cutting  force  diagram 
according  to  Ernst  and 
Merchant 


340  9      Physics  of  Metal  Cutting 


By  employing  simple  mechanics,  the  force  can  be  resolved  into  two  perpendicu- 
lar components,  Fc  and  F,.  As  can  be  seen  in  Figure  9.12,  Fc  acts  in  the  direction  of 
tool  travel  and  is  referred  to  as  the  cutting  component,  whereas  F,  acts  normal  to  that 
direction  and  is  known  as  the  thrust  component.  The  resultant  tool  force  can  alterna- 
tively be  resolved  into  another  two  perpendicular  components,  Fs  and  Fn.  The  first 
component,  Fs,  acts  along  the  shear  plane  and  is  referred  to  as  the  shearing  force;  the 
second  component,  Fn,  acts  normal  to  it  and  causes  compressive  stress  to  act  on  the 
shear  plane.  Again,  at  the  chip-tool  interface,  the  components  of  the  resultant  force 
that  acts  on  the  chip  are  F  and  TV.  Notice  from  the  figure  that  F  represents  the  fric- 
tion force  that  resists  the  movement  of  the  chip  as  it  slides  over  the  face  of  the  tool, 
while  N  is  the  normal  force.  The  ratio  between  F  and  TV  is  actually  the  coefficient  of 
friction  at  the  chip-tool  interface.  Because  each  two  components  are  perpendicular,  it 
is  clear  from  Euclidean  geometry  that  the  point  of  intersection  of  each  two  compo- 
nents must  lie  on  the  circumference  of  the  circle  that  has  the  resultant  force  as  a  di- 
ameter. The  cutting  force  diagram  of  Figure  9.12  lets  us  express  Fs,  Fn,  F,  and  TV  in 
terms  Fc  and  F,  as  follows: 

Fs  =  Fc  cos  §  -  F,  sin  (J)  (9.6) 

Fn  =  Fc  sin  <J)  +  F,  cos  (J)  (9.7) 

F  =  Fc  sin  a  +  Ft  cos  a  (9.8) 

N  =  Fc  cos  a  -  F,  sin  a  (9.9) 

The  preceding  equations  can  be  used  to  determine  different  unknown  parameters 
that  affect  the  cutting  operations.  For  instance,  the  coefficient  of  friction  at  the  chip- 
tool  interface  can  be  obtained  as  follows: 

F      F,.  sin  cl  +  F,  cos  a  _i  0 

u.  =  —  =  — - - =  tan     B 

TV      Fc  cos  a-  Ft  sin  a 

Dividing  both  the  numerator  and  denominator  by  cos  a,  we  obtain 

F,  +  F,tana  (9>10) 

Fc  -  F,  tan  a 

The  shear  force  Fs  is  of  particular  importance  as  it  is  used  for  obtaining  the  mag- 
nitude of  the  mean  shear  strength  of  the  material  along  the  shear  plane  and  during  the 
cutting  operation.  This  is  equal  to  the  mean  shear  stress  acting  through  the  shear  plane 
and  can  be  computed  as  follows: 

'     A, 

where  As,  the  area  of  the  shear  plane,  equals  Achip/sm  ((),  where  Achip  is  the  cross- 
sectional  area  of  the  chip.  Therefore, 

x  _  [Fc  cos  (()  -  F,  sin  <t>]sin  ()) 

•™chip 


9.3  Cutting  Forces  341 

Experimental  work  has  indicated  that  the  mean  shear  stress,  calculated  from 
Equation  9.11,  is  constant  for  a  given  metal  over  a  wide  variation  in  the  cutting  con- 
ditions. This  can  be  explained  by  the  fact  that  the  strain  rate  at  which  metal  cutting 
occurs  is  sufficiently  high  to  be  the  only  factor  that  affects  the  shear  strength  for  a 
given  material.  Therefore,  the  cutting  speed,  amount  of  strain,  or  temperature  do  not 
have  any  appreciable  effect  on  the  value  of  the  mean  shear  stress  of  the  metal  being 
machined. 

Ernst  and  Merchant  extended  their  analysis  and  studied  the  relationship  between 
the  shear  angle  and  the  cutting  conditions.  They  suggested  that  the  shear  angle  always 
takes  the  value  that  reduces  the  total  energy  consumed  in  cutting  to  a  minimum.  Be- 
cause the  total  work  done  in  cutting  is  dependent  upon  and  is  a  direct  function  of  the 
component  Fc  of  the  cutting  force,  they  developed  an  expression  for  Fc  in  terms  of  (J) 
and  the  constant  properties  of  the  workpiece  material.  Next,  that  expression  was  dif- 
ferentiated with  respect  to  <J)  and  then  equated  to  zero  in  order  to  obtain  the  value  §  for 
which  Fc  and,  therefore,  the  energy  consumed  in  cutting  is  a  minimum.  Following  is 
the  mathematical  treatment  of  this  problem. 

From  Figure  9.12,  we  can  see  that 

Fs  =  R  cos  (4>  +  p  -  a)  (9.12) 

Therefore, 

cos  (§  +  p  -  a) 
But, 


Fs  =  xsAs  =  xs 


'chip 

sin  § 
Therefore, 


R  =  T*AchiP  x (9.13) 

sin  §       cos  (<|)  +  P  -  a) 

Again,  it  can  be  seen  from  Figure  9. 1 2  that 

Fc  =  /?cos(p-<x)  (9.14) 

Hence,  from  Equations  9.13  and  9.14, 

F  =  TAhiD  x     cos  (p  -  a)  (9lS) 

sin  §       cos  ((()  +  P  -  a) 

Differentiating  Equation  9.15  with  respect  to  ({)  and  equating  the  outcome  to  zero,  we 
obtain  the  condition  that  will  make  Fc  minimal.  This  condition  is  given  by  the  follow- 
ing equation: 

20  +  P  -  a  =  ^  (9.16) 


342  9      Physics  of  Metal  Cutting 


It  was  found  that  the  theoretical  value  of  ty  obtained  from  Equation  9.16  agreed  well 
with  the  experimental  results  when  cutting  polymers,  but  this  was  not  the  case  when 
machining  aluminum,  copper,  or  steels. 

Theory  of  Lee  and  Shaffer 

The  theory  of  American  manufacturing  scientists  E.  Lee  and  Bernard  W.  Shaffer  is 
based  on  applying  the  slip-line  field  theory  to  the  two-dimensional  metal-cutting  prob- 
lem. A  further  assumption  is  that  the  material  behaves  in  a  rigid,  perfectly  plastic  man- 
ner and  obeys  the  von  Mises  yield  criterion  and  its  associated  flow  rule.  After 
constructing  the  slip-line  field  for  that  problem,  it  was  not  difficult  for  Lee  and  Shaf- 
fer to  obtain  the  relationship  between  the  cutting  parameters  and  the  shear  angle.  The 
result  can  be  given  by  the  following  equation: 

<|>  +  f3  -  a  =  -j  (9.17) 

In  fact,  neither  of  the  preceding  theories  quantitatively  agrees  with  experimental  re- 
sults. However,  the  theories  yield  linear  relationships  between  ())  and  ((3  -  a),  which  is 
qualitatively  in  agreement  with  the  experimental  results. 

Cutting  Energy 

We  can  see  from  the  previous  discussion  that  it  is  the  component  Fc  that  determines 
the  energy  consumed  during  machining  because  it  acts  along  the  direction  of  relative 
tool  travel.  The  power  consumption  P,„  (i.e.,  the  rate  of  energy  consumption  during 
machining)  can  be  obtained  from  the  following  equation: 

Pm  =FcxV  (9.18) 

where  V  is  the  cutting  speed. 

The  rate  of  metal  removal  during  machining  Zm  is  also  proportional  to  the  cutting 
speed  and  can  be  given  by 

Zm  =A0xV  (9.19) 

where  A0,  the  cross-sectional  area  of  the  uncut  chip,  equals  t0  times  the  width  of  the 
chip.  Now,  the  energy  consumed  in  removing  a  unit  volume  of  metal  can  be  obtained 
from  Equations  9.18  and  9.19  as  follows: 

n       P,„      Fc  x  V      Fc 

P(  =  —  =  — =  —  (9.20) 

Zm      At)  x  V      A„ 

In  Equation  9.20,  Pc,  a  parameter  that  indicates  the  efficiency  of  the  process,  is 
commonly  known  as  the  specific  cutting  energy  and  also  sometimes  is  called  the 
unit  horsepower.  Unfortunately,  the  specific  cutting  energy  for  a  given  metal  is  not 
constant  but  rather  varies  considerably  with  the  cutting  conditions,  as  we  will  see 
later. 


9.4  Oblique  Versus  Orthogonal  Cutting 


343 


BLIQUE  VERSUS  ORTHOGONAL  CUTTING 


Until  now,  we  have  simplified  the  metal-cutting  process  by  considering  only  orthogo- 
nal cutting.  In  this  type  of  cutting,  the  cutting  edge  of  the  tool  is  normal  to  the  direc- 
tion of  relative  tool  movement,  as  shown  in  Figure  9.13a.  It  is  actually  a 
two-dimensional  process  in  which  each  longitudinal  section  (i.e.,  parallel  to  the  tool 
travel)  of  the  tool  and  chip  is  identical  to  any  other  longitudinal  section  of  the  tool  and 
chip.  The  cutting  force  is,  therefore,  also  two-dimensional  and  can  be  resolved  into  two 
components,  both  lying  within  the  plane  of  the  drawing.  Although  this  approach  facil- 
itated the  analysis  of  chip  formation  and  the  mechanics  of  metal  cutting,  it  is  seldom 
used  in  practice  because  it  applies  only  when  turning  the  end  face  of  a  thin  tube  in  a 
direction  parallel  to  its  axis. 

The  more  common  type  (or  model)  of  cutting  used  in  the  various  machining  op- 
erations is  oblique  cutting.  In  this  case,  the  cutting  edge  of  the  tool  is  inclined  to  (i.e., 
not  normal  to)  the  relative  tool  travel,  as  can  be  seen  in  Figure  9.13b.  It  is  a  three- 
dimensional  problem  in  which  the  cutting  force  can  be  resolved  into  three  perpendic- 
ular components,  as  indicated  in  Figure  9.14.  The  magnitudes  of  these  components  can 
be  measured  by  means  of  a  special  apparatus  that  is  mounted  either  in  the  workholder 
or  toolholder  and  is  known  as  a  dynamometer.  As  you  may  expect,  the  tool  geometry 


FIGURE    9.13 

Types  of  cutting:  (a) 
orthogonal;  (b)  oblique 


(a) 


(b) 


FIGURE    9.14 

Components  of  the 
three-dimensional 
cutting  force 


Tool 


344  9      Physics  of  Metal  Cutting 


is  rather  complicated  and  will  be  discussed  later.  For  now,  let  us  see  the  effect  of  each 
of  the  cutting  force  components  on  the  oblique  cutting  operations. 

Forces  in  Oblique  Cutting 

Following  is  a  discussion  of  the  three  components  referred  to  as  Fc,  Ff,  and  Fr  in  Fig- 
ure 9.14: 

1.  Fc  is  the  cutting  force  and  acts  in  the  direction  where  the  cutting  action  takes  place. 
It  is  the  highest  of  the  three  components  and  results  in  99  percent  of  the  energy  con- 
sumed during  the  process.  The  horsepower  due  to  this  force,  hpc,  can  be  given  by 
the  following  equation: 

FcxVc 

hp-=l^  <9-21) 

In  Equation  9.21,  Fc  is  in  pounds  and  Vc  is  in  feet  per  minute.  Consequently,  the 
appropriate  conversion  factors  must  be  used  if  the  horsepower  is  to  be  obtained  in 
SI  units. 

2.  Ff  is  the  feed  force  (or  longitudinal  force  in  turning).  The  term  feed  means  the 
movement  of  the  tool  to  regenerate  the  cutting  path  in  order  to  obtain  the  machined 
surface.  This  force  amounts  to  only  about  40  percent  of  the  cutting  force.  The 
horsepower  required  to  feed  the  tool,  hpf,  can  be  given  as  follows: 

Ffx  Vf 

hpf  =  t?, 7-  (9.22) 

Ff      550x60 

The  horsepower  given  by  Equation  9.22  amounts  to  only  1  percent  of  the  total 
power  consumed  during  cutting. 

3.  Fr  is  the  thrust  force  (or  radial  force  in  turning)  and  acts  in  the  direction  of  the 
depth  of  cutting.  This  force  is  the  smallest  of  the  three  components  and  amounts  to 
only  20  percent  of  the  cutting  force  or,  in  other  words,  50  percent  of  the  feed  force. 
This  component  does  not  result  in  any  power  consumption  as  there  is  no  tool  move- 
ment along  the  direction  of  the  depth  of  cut. 

These  components  of  the  cutting  force  are  measured  only  in  scientific  metal- 
cutting  research.  The  manufacturing  engineer  is,  however,  interested  in  determining 
beforehand  the  motor  horsepower  required  to  perform  a  certain  job  in  order  to  be  able 
to  choose  the  right  machine  for  that  job.  Therefore,  use  is  made  of  the  concept  of  unit 
horsepower,  which  was  mentioned  previously.  Experimentally  obtained  values  of  unit 
horsepower  for  various  common  materials  are  compiled  in  tables  ready  for  use.  The 
total  cutting  horsepower  can  be  obtained  from  the  following  equation: 

hpc  =  unit  hp  x  rate  of  metal  removal  x  correction  factor  (9.23) 

where  the  rate  of  metal  removal  is  in  cubic  inches  per  minute  and  the  correction  fac- 
tor is  introduced  to  account  for  the  tool  geometry  and  and  the  variation  in  feed. 


9.4  Oblique  Versus  Orthogonal  Cutting 


345 


Table  9.1  indicates  the  unit  horsepower  values  for  various  ferrous  metals  and  al- 
loys having  different  hardness  numbers.  Table  9.2  provides  the  unit  horsepower  values 
for  nonferrous  metals  and  alloys.  Figure  9.15a  through  c  indicates  the  different  correc- 
tion factors  for  the  unit  horsepower  to  account  for  variations  in  the  cutting  conditions. 

The  cutting  horsepower  is  not  of  practical  importance  by  itself.  Its  significance  is 
that  it  is  used  in  computing  the  motor  horsepower.  Obviously,  the  motor  horsepower 


TABLE    9.1 

Unit  horsepower  values  for  ferrous  metals  and  alloys 


Brinnei  Hardness  Number 


Ferrous 

Metals  and 

Alloys 

150-175 

176-200 

201-250 

251-300 

301-350 

351-400 

ANSI 

1010-1025 

.58 

.67 

— 

— 

— 

— 

1030-1055 

.58 

.67 

.80 

.96 

— 

— 

1060-1095 

— 

— 

.75 

.88 

1.0 

— 

1112-1120 

.50 

— 

— 

— 

— 

— 

1314-1340 

.42 

.46 

.50 

— 

— 

— 

1330-1350 

— 

.67 

.75 

.92 

1.1 

— 

2015-2115 

.67 

— 

— 

— 

— 

— 

2315-2335 

.54 

.58 

.62 

.75 

.92 

1.0 

2340-2350 

— 

.50 

.58 

.70 

.83 

— 

2512-2515 

.50 

.58 

.67 

.80 

.92 

— 

3115-3130 

.50 

.58 

.70 

.83 

1.0 

1.0 

3160-3450 

— 

.50 

.62 

.75 

.87 

1.0 

4130-4345 

— 

.46 

.58 

.70 

.83 

1.0 

4615-4820 

.46 

.50 

.58 

.70 

.83 

.87 

5120-5150 

.46 

.50 

.62 

.75 

.87 

1.0 

52100 

— 

.58 

.67 

.83 

1.0 

— 

6115-6140 

.46 

.54 

.67 

.83 

1.0 

— 

6145-6195 

— 

.70 

.83 

1.0 

1.2 

1.3 

Plain  cast  iron 

.30 

.33 

.42 

.50 

— 

— 

Alloy  cast  iron 

.30 

.42 

.54 

— 

— 

— 

Malleable  iron 

.42 

— 

— 

— 

— 

— 

Cast  steel 

.62 

.67 

.80 

— 

— 

— 

Source:  Turning  Handbook  of  High-Efficiency  Metal  Cutting,  7950,  courtesy  Carboloy  Inc.,  a  Seco  Tools  Company. 


346 


9      Physics  of  Metal  Cutting 


TABLE    9.2 

Unit  horsepower  values 
for  nonferrous  metals 
and  alloys 


Nonferrous  Metals  and  Alloys 

Properties 

Unit  Horsepower 

Brass 

Hard 

0.83 

Medium 

0.50 

Soft 

0.33 

Free  machining 

0.25 

Bronze 

Hard 

0.83 

Medium 

0.50 

Soft 

0.33 

Copper 

Pure 

0.90 

Aluminum 

Cast 

0.25 

Hard  (rolled) 

0.33 

Monel 

(rolled) 

1.0 

Zinc  alloy 

(die  cast) 

0.25 

Source:  Turning  Handbook  of  High-Efficiency  Metal  Cutting,  1980,  courtesy  Carboloy  Inc.,  A  Seco  Tools 
Company. 


has  to  be  higher  than  the  cutting  horsepower  as  some  power  is  lost  in  overcoming  fric- 
tion and  inertia  of  the  moving  parts.  The  following  equation  can  be  used  for  calculat- 
ing the  motor  horsepower: 


hpm  =  hpc  x 


1 


(9.24) 


where  r\  is  the  machine  efficiency,  which  can  be  taken  from  Table  9.3. 

The  cutting  horsepower  is  used  not  only  in  calculating  the  motor  horsepower 
but  also  for  giving  a  fair  estimate  of  the  cutting  force  component  Fc  by  using  Equa- 
tion 9.21.  This  force  is  very  important  when  studying  the  vibrations  associated  with 
metal  cutting,  as  we  will  see  later.  The  following  example  illustrates  how  to  estimate 
the  cutting  force  component. 


Example  of  Estimating  Cutting 
Force  Component 

During  a  turning  operation,  the  metal-removal  rate  (M.R.R.)  was  found  to  be  3.6  cubic 
inches  per  minute.  Following  are  other  data  of  the  process: 


Material: 
Cutting  speed: 
Undeformed  chip  thickness: 
Tool  character: 

Estimate  the  cutting  force  component  Fc 


ANSI  1055,  HB  250 

300  feet  per  minute 

0.01  inch 

0-7-7-7-15-15-1/32  (see  Section  9.5) 


9.4  Oblique  Versus  Orthogonal  Cutting 


347 


FIGURE    9.15 

Different  correction 
factors  to  account  for 
variations  in  the  cutting 
conditions:  (a)  cutting 
speed;  (b)  chip 
thickness;  (c)  rake 
angle  (Source:  Turning 
Handbook  of  High- 
Efficiency  Metal 
Cutting,  1980,  courtesy 
Carboloy  Inc.,  A  Seco 
Tools  Company) 


1.6 
1.2 
0.8 
0.4 


200       400       600       800 
Cutting  speed  (SFPM) 

(a) 


0.0010.002  0.004     0.010  0.020  0.040      0.100 
Undeformed  chip  thickness  (in.) 


(b) 


cr 

1.4 

- 

1.2 

\ 

1.0 

\. 

0.8 

N 

^. 

0.6 

' 

i 

i 

-20     -10       0       +10     +20 
True  rake  angle 


Here  is  the  solution  to  this  example  problem: 

spindle  hp  =  M.R.R.  x  unit  hp  x  correction  factor 

The  correction  factor  because  of  the  cutting  speed  is  0.8,  and  the  correction  factor  be- 
cause of  the  undeformed  chip  thickness  is  1 .  The  true  rake  angle  is 

tan  a,™  =  cos  15°  tan  7°  +  sin  15°  tan  0 


TABLE    9.3 

r 

-\ 

Typical  overall  machine 

Type 

Efficiency  (%) 

tool  efficiencies  (except 

milling  machines) 

Direct-spindle  drive 

90 

One-belt  drive 

85 

Two-belt  drive 

70 

Geared  head 

70 

Source:  Turning  Handbook  of  High-Efficiency  Metal 
Cutting,  1980,  courtesy  Carboloy  Inc.,  A  Seco  Tools 
Company. 


348 


Physics  of  Metal  Cutting 


where  atrue  =  6°  (see  Section  9.5).  The  correction  factor  because  of  the  true  rake  angle 
is  0.83,  and  the  unit  hp  is  0.8  from  Table  9.1.  Thus, 

spindle  hp  =  3.6  x  0.8  x  0.8  x  1  x  0.83  =  1.9  hp 

Fr  x  300  ft/min. 


hpc  = 


Therefore, 


550  x  60 


„       1.9x550x60 

Fc  = — =  209  pounds 

300  F 

Note  that  the  undeformed  chip  thickness  equals  feed  (inches  per  revolution)  times  the 
cosine  of  the  side  cutting-edge  angle. 


9.5  CUTTING  TOOLS 

Basic  Geometry 


In  order  for  a  tool  to  cut  a  material,  it  must  have  two  important  characteristics:  First,  it 
must  be  harder  than  that  material,  and,  second,  it  must  possess  certain  geometrical 
characteristics.  The  cutting  tool  geometry  differs  for  different  machining  operations. 
Nevertheless,  it  is  always  a  matter  of  rake  and  clearance  angles.  Therefore,  we  are 
going  to  limit  our  discussion,  at  the  moment,  to  single-point  tools  for  the  sake  of  sim- 
plicity. Other  types  of  tools  will  be  considered  when  we  cover  the  various  machining 
operations. 

As  can  be  seen  in  Figure  9.16,  the  geometry  of  a  single-point  cutting  tool  can  be 
adequately  described  by  six  cutting  angles.  These  can  be  shown  more  clearly  by  pro- 
jecting them  on  three  perpendicular  planes  using  orthogonal  projection,  as  is  done  in 
Figure  9.17.  Let  us  now  consider  the  definition  of  each  of  the  six  angles. 

Side  cutting-edge  angle.  The  side  cutting-edge  angle  (SCEA)  is  usually  referred  to  as 
the  lead  angle.  It  is  the  angle  enclosed  between  the  side  cutting  edge  and  the  longitu- 
dinal direction  of  the  tool.  The  value  of  this  angle  varies  between  0°  and  90°,  depend- 
ing upon  the  machinability,  rigidity,  and,  sometimes,  the  shape  of  the  workpiece  (e.g., 


FIGURE    9.16 

Geometry  of  a  single- 
point  cutting  tool 


End  relief 


9.5  Cutting  Tools 


349 


FIGURE    9.17 

Orthogonal  projection 
of  the  cutting  angles  of 
a  single-point  tool  and 
tool  character 


ECEA  20° 

Nose  radius 


SCEA  15° 
Top  view 


Side  rake  8° 


Back  rake  2° 


Side  relief  6 


Side  view 
(dotted  lines  are  not  shown) 


[*  End  relief  6° 
Front  view 


Tool  character 
2°     8°     6°     6 


Back  rake- 
Side  rake- 


End  relief- 
Side  relief - 
ECEA 


20°      15°     ±\n. 


SCEA- 


Nose  radius 


a  90°  shoulder  must  be  produced  by  a  0°  SCEA).  As  this  angle  increases  from  0°  to 
15°,  the  power  consumption  during  cutting  decreases.  However,  there  is  a  limit  for  in- 
creasing the  SCEA,  beyond  which  excessive  vibrations  take  place  because  of  the  large 
tool-workpiece  interface.  On  the  other  hand,  if  the  angle  were  taken  as  0°,  the  full  cut- 
ting edge  would  start  to  cut  the  workpiece  at  once,  causing  an  initial  shock.  Usually, 
the  recommended  value  for  the  lead  angle  should  range  between  15°  and  30°. 

End  cutting-edge  angle.  The  end  cutting-edge  angle  (ECEA)  serves  to  eliminate  rub- 
bing between  the  end  cutting  edge  and  the  machined  surface  of  the  workpiece.  Al- 
though this  angle  takes  values  in  the  range  of  5°  to  30°,  commonly  recommended 
values  are  8°  to  15°. 

Side  relief  and  end  relief  angles.  Side  and  end  relief  angles  serve  to  eliminate  rub- 
bing between  the  workpiece  and  the  side  and  end  flank,  respectively.  Usually,  the  value 
of  each  of  these  angles  ranges  between  5°  and  15°. 

Back  and  side  rake  angles.  Back  and  side  rake  angles  determine  the  direction  of  flow 
of  the  chips  onto  the  face  of  the  tool.  Rake  angles  can  be  positive,  negative,  or  zero.  It 
is  the  side  rake  angle  that  has  the  dominant  influence  on  cutting.  Its  value  usually 
varies  between  0°  and  15°,  whereas  the  back  rake  angle  is  usually  taken  as  0°. 

Another  useful  term  in  metal  cutting  is  the  true  rake  angle,  which  is  confined  be- 
tween the  line  of  major  inclination  within  the  face  of  the  tool  and  a  horizontal  plane. 
It  determines  the  actual  flow  of  chips  across  the  face  of  the  tool  and  can  be  obtained 


350  9      Physics  of  Metal  Cutting 


from  the  following  equation: 

true  rake  angle  =  tan" '(tan  a  sin  X  +  tan  f3  cos  X)  (9.25) 

where:  a  is  the  back  rake  angle 
(3  is  the  side  rake  angle 
X  is  the  lead  angle  (SCEA) 

As  previously  mentioned,  the  true  rake  angle  has  a  marked  effect  on  the  unit  horse- 
power for  a  given  workpiece  material,  and  a  correction  factor  has  to  be  used  when  cal- 
culating the  power  in  order  to  account  for  variations  in  the  true  rake  angle. 

Tool  character.  The  tool  angles  are  usually  specified  by  a  standard  abbreviation  sys- 
tem called  the  tool  character,  or  the  tool  signature.  As  also  illustrated  in  Figure  9.17, 
the  tool  angles  are  always  given  in  a  certain  order:  back  rake,  side  rake,  end  relief,  side 
relief,  ECEA,  and  SCEA,  followed  by  the  nose  radius  of  the  tool. 

Cutting  Tool  Materials 

Cutting  tools  must  possess  certain  mechanical  properties  in  order  to  function  ade- 
quately during  the  cutting  operations.  These  properties  include  high  hardness  and  the 
ability  to  retain  it  even  at  the  elevated  temperatures  generated  during  cutting,  as  well 
as  toughness,  creep  and  abrasion  resistance,  and  the  ability  to  withstand  high  bearing 
pressures.  Cutting  materials  differ  in  the  degree  to  which  they  possess  each  of  these 
mechanical  properties.  Therefore,  a  cutting  material  is  selected  to  suit  the  cutting  con- 
ditions (i.e.,  the  workpiece  material,  cutting  speed  or  production  rate,  coolants  used, 
and  so  on).  Following  is  a  survey  of  the  commonly  used  cutting  tool  materials. 

Plain-carbon  steel.  Plain-carbon  steel  contains  from  0.8  to  1.4  percent  carbon,  has  no 
additives,  and  is  subjected  to  heat  treatment  to  increase  its  hardness.  Plain-carbon  steel 
is  suitable  only  when  making  hand  tools  or  when  soft  metals  are  machined  at  low  cut- 
ting speeds  as  it  cannot  retain  its  hardness  at  temperatures  above  600°F  (300°C)  due  to 
tempering  action. 

Alloy  steel.  The  carbon  content  of  alloy  steel  is  similar  to  that  of  plain-carbon  steel, 
but  it  contains  alloying  elements  (in  limited  amounts).  Tools  made  of  alloy  steel  must 
be  heat  treated  and  are  used  only  when  machining  is  carried  out  at  low  cutting  speeds. 
The  temperature  generated  as  a  result  of  cutting  should  not  exceed  600°F  (300°C)  to 
avoid  any  tempering  action. 

High-speed  steel.  High-speed  steel  (HSS)  is  a  kind  of  alloy  steel  that  contains  a  cer- 
tain percentage  of  alloying  elements,  such  as  tungsten  (18  percent),  chromium  (4  per- 
cent), molybdenum,  vanadium,  and  cobalt.  High-speed  steel  is  heat  treated  by  heating 
(at  two  stages),  cooling  by  employing  a  stream  of  air,  and  then  tempering  it.  Tools 
made  of  HSS  can  retain  their  hardness  at  elevated  temperatures  up  to  1 100°F  (600°C). 
These  tools  are  used  when  relatively  high  cutting  speeds  are  required.  Single-point 
tools,  twist  drills,  and  milling  cutters  are  generally  made  of  high-speed  steel,  except 
when  these  tools  are  required  for  high-production  machining. 


9.5  Cutting  Tools  351 

Cast  hard  alloys.  Cast  hard  alloys  can  be  either  ferrous  or  nonferrous  and  contain 
about  3  percent  carbon,  which,  in  turn,  reacts  with  the  metals  to  form  very  hard  car- 
bides. The  carbides  retain  their  hardness  even  at  a  temperature  of  about  1650°F 
(900°C).  Because  such  a  material  cannot  be  worked  or  machined,  it  is  cast  in  ceramic 
molds  to  take  the  form  of  tips  that  are  mounted  onto  holders  by  brazing  or  by  being 
mechanically  fastened. 

Sintered  cemented-carbide  tips.  Sintered  cemented  carbide  was  developed  to  elimi- 
nate the  main  disadvantage  of  the  hard  cast  alloys:  brittleness.  Originally,  the  compo- 
sition of  this  material  involved  about  82  percent  very  hard  tungsten  carbide  particles 
and  18  percent  cobalt  as  a  binder.  Sintered  cemented  carbides  are  always  molded  to 
shape  by  the  powder  metallurgy  technique  (i.e.,  pressing  and  sintering,  as  was  ex- 
plained in  Chapter  7).  As  it  is  impossible  to  manufacture  the  entire  tool  out  of  ce- 
mented carbide  because  of  the  strength  consideration,  only  tips  are  made  of  this 
material;  these  tips  are  brazed  or  mechanically  fastened  to  steel  shanks  that  have  the 
required  cutting  angles. 

Cemented  carbides  used  to  be  referred  to  as  Widia,  taken  from  the  German  ex- 
pression "Wie  Diamant,"  meaning  diamondlike,  because  they  possess  extremely  high 
hardness,  reaching  about  90  Re,  and  they  retain  such  hardness  even  at  temperatures  of 
up  to  1850°F  (1000°C).  Recent  developments  involve  employing  combinations  of 
tungsten,  titanium,  and  tantalum  carbides  with  cobalt  or  nickel  alloy  as  binders.  The  re- 
sult is  characterized  by  its  low  coefficient  of  friction  and  high  abrasion  resistance. 
Tools  with  cemented-carbide  tips  are  recommended  whenever  the  cutting  speeds  re- 
quired or  the  feed  rates  are  high  and  are,  therefore,  commonly  used  in  mass  produc- 
tion. Recently,  carbide  tips  have  been  coated  with  nitrites  or  oxides  to  increase  their 
wear  resistance  and  service  life. 

Ceramic  tips.  Ceramic  tips  consist  basically  of  very  fine  alumina  powder,  A1203, 
which  is  molded  by  pressing  and  sintering.  Ceramics  have  almost  the  same  hardness 
as  cemented  carbides,  but  they  can  retain  that  hardness  up  to  a  temperature  of  2200°F 
(1100°C)  and  have  a  very  low  coefficient  of  thermal  conductivity.  Such  properties 
allow  for  cutting  to  be  performed  at  speeds  that  range  from  two  to  three  times  the  cut- 
ting speed  used  when  carbide  tips  are  employed.  Ceramic  tips  are  also  characterized  by 
their  superior  resistance  to  wear  and  to  the  formation  of  crater  cavities.  They  require 
no  coolants.  Their  toughness  and  bending  strength  are  low,  which  must  be  added  to 
their  sensitivity  to  creep  loading  and  vibration.  Therefore,  ceramic  tips  are  recom- 
mended only  for  finishing  operations  (small  depth  of  cut)  at  extremely  high  cutting 
speeds  of  up  to  180  feet  per  minute  (600  m/min.).  Following  are  the  three  common 
types  of  ceramic  tips: 

1.  Oxide  tips,  consisting  mainly  of  aluminum  oxide,  have  a  white  color  with  some 
pink  or  yellow  tint. 

2.  Cermet  tips,  including  alumina  and  some  metals  such  as  titanium  or  molybdenum, 
are  dark  gray  in  color. 

3.  Tips  that  consist  of  both  oxides  as  well  as  carbides  are  black  in  color. 


352  9      Physics  of  Metal  Cutting 


Ceramic  tips  should  not  be  used  for  machining  aluminum  because  of  their  affinity  to 
oxygen. 

Diamond.  Diamond  pieces  are  fixed  to  steel  shanks  and  are  used  in  precision  cutting 
operations.  They  are  recommended  for  machining  aluminum,  magnesium,  titanium, 
bronze,  rubber,  and  polymer.  When  machining  metallic  materials,  a  mirror  finish  can 
be  obtained. 

Tool  Wear 

There  are  two  interrelated  causes  for  tool  wear:  mechanical  abrasion  and  thermal  ero- 
sion. Although  these  two  actions  take  place  simultaneously,  the  role  of  each  varies  for 
various  cutting  conditions.  Mechanical  wear  is  dominant  when  low  cutting  speeds  are 
used  or  when  the  workpiece  possesses  high  machinability.  Thermal  wear  prevails  when 
high  cutting  speeds  are  used  with  workpieces  having  low  machinability.  Thermal  wear 
is  due  to  diffusion,  oxidation,  and  the  fact  that  the  mechanical  properties  of  the  tool 
change  as  a  result  of  the  high  temperature  generated  during  the  cutting  operation. 

The  face  of  the  cutting  tool  is  subjected  to  friction  caused  by  the  fast  relative 
motion  of  the  generated  chips  onto  its  surface.  Similarly,  the  flanks  are  also  subjected 
to  friction  as  a  result  of  rubbing  by  the  workpiece.  Although  the  tool  is  harder  than 
the  workpiece,  friction  and  wear  will  take  place  and  will  not  be  evenly  distributed 
over  the  face  of  the  tool.  Wear  is  localized  in  the  vicinity  of  the  cutting  edge  and  re- 
sults in  the  formation  of  a  crater.  There  are  different  kinds  of  tool  wear: 

1.  Flank  wear 

2.  Wear  of  the  face  that  comes  in  contact  with  the  removed  chip 

3.  Wear  of  the  cutting  edge  itself 

4.  Wear  of  the  nose 

5.  Wear  and  formation  of  a  crater 

6.  Cracks  in  the  cutting  edges  occurring  during  interrupted  machining  operations 
such  as  millings 

Tool  Life 

Tool  life  is  defined  as  the  length  of  actual  machining  time  beginning  at  the  moment 
when  a  just-ground  tool  is  used  and  ending  at  the  moment  when  the  machining  opera- 
tion is  stopped  because  of  the  poor  performance  of  that  tool.  Different  criteria  can  be 
used  to  judge  the  moment  at  which  the  machining  operation  should  be  stopped.  It  is 
common  to  consider  the  tool  life  as  over  when  the  flank  wear  reaches  a  certain  amount 
(measured  as  the  length  along  the  surface  generated  due  to  abrasion  starting  from  the 
tip).  This  maximum  permissible  flank  wear  is  taken  as  0.062  inch  (1.58  mm)  in  the 
case  of  high-speed  steel  tools  and  0.03  inch  (0.76  mm)  for  carbide  tools. 

The  tool  life  is  affected  by  several  variables,  the  important  ones  being  cutting 
speed,  feed,  and  the  coolants  used.  The  effect  of  these  variables  can  be  determined  ex- 
perimentally and  then  represented  graphically  for  practical  use.  It  was  found  by  Fred- 
erick W.  Taylor  that  the  relationship  between  tool  life  and  cutting  speed  is  exponential. 
It  can,  therefore,  be  plotted  on  a  logarithmic  scale  so  that  it  takes  the  form  of  a  straight 


9.6  Machinability 


353 


FIGURE    9.18 

Relationship  between 
tool  life  and  cutting 
speed  on  a  log-log 
scale 


Tool  life,  min.  (log  scale) 


line,  as  shown  in  Figure  9.18.  In  fact,  this  was  the  basis  for  establishing  an  empirical 
formula  that  correlates  tool  life  with  cutting  speed.  A  correction  factor  is  also  intro- 
duced into  the  formula  to  account  for  the  effects  of  other  variables.  The  original  for- 
mula had  the  following  form: 


VTn  =  c 


(9.26) 


where:  n  is  a  constant  that  depends  upon  the  tool  material  (0. 1  for  HSS,  0.20  for 
carbides,  and  0.5  for  ceramic  tools) 

c  is  a  constant  that  depends  upon  the  cutting  conditions  (e.g.,  feed) 

T  is  the  tool  life  measured  in  minutes 

V  is  the  cutting  speed  in  feet  per  minute 

Equation  9.26  is  very  useful  in  obtaining  the  tool  life  for  any  cutting  speed  if  the  tool 
life  is  known  at  any  other  cutting  speed. 


9.6  MACHINABILITY 


Machinability  Defined 

Machinability  is  a  property  characterizing  the  material  of  the  workpiece:  It  is  the  ease 
with  which  that  material  can  be  machined.  In  order  to  express  machinability  in  a  quan- 
titative manner,  one  of  the  following  methods  is  used: 

1.  The  maximum  possible  rate  of  chip  removal 

2.  Surface  finish  of  the  machined  workpiece 

3.  Tool  life 

4.  Energy  required  to  accomplish  the  cutting  operation 

It  is  clear  that  the  tool  life  is  the  most  important  of  these  criteria  as  it  plays  an  impor- 
tant role  in  maximizing  the  production  while  minimizing  the  production  cost.  More- 
over, criteria  such  as  surface  finish  and  machining  precision  depend  upon  many 
factors,  such  as  the  sharpness  of  the  cutting  edge,  the  rigidity  of  the  tool,  and  the  pos- 


354 


Physics  of  Metal  Cutting 


TABLE    9.4 

Machinability  indices 
for  some  metals  and 
alloys  (using  carbide 
tools) 


Machinability 

Metal  or  Alloy 

Index  (%) 

Steel  SAE  1020  (annealed) 

65 

Steel  SAE  A2340 

45 

Cast  iron 

70-80 

Stainless  steel  18-8  (austenitic) 

25 

Tool  steel  (low  tungsten,  chrome,  and  carbon) 

30 

Copper 

70 

Brass 

180 

Aluminum  alloy 

300  and  above 

sibility  of  formation  of  a  built-up  edge.  As  a  consequence,  it  is  the  tool  life  that  is  most 
suitable  as  a  criterion  of  machinability. 

Machinability  Index 

Because  machinability  cannot  be  expressed  in  an  absolute  manner,  it  is  appropriate  to 
take  a  highly  machinable  metal  as  a  reference  and  express  the  machinability  of  any 
other  ferrous  metal  as  a  percentage  of  that  of  the  reference  metal.  The  reference  metal 
chosen  was  steel  SAE-AISI  1112  because  of  its  superior  machinability,  which  exceeds 
that  for  any  other  steel.  Such  steel  is  usually  referred  to  as  free  cutting  steel.  The 
machinability  index  can  now  be  given: 


machinability  index  = 


cutting  speed  of  metal  for  tool  life  of  20  minutes 
cutting  speed  of  steel  SAE  1 1 1 2  for  tool  life  of  20  minutes 


x  100    (9.27) 


Table  9.4  indicates  the  machinability  index  for  some  commonly  used  metals  and 
alloys. 


JUTTING  FLUIDS 


Necessary  Characteristics 

As  previously  mentioned,  the  process  of  metal  cutting  results  in  the  generation  of  a 
large  amount  of  heat  and  a  localized  increase  in  the  temperature  of  the  cutting  tool. 
This  effect  is  particularly  evident  when  machining  ductile  metals.  Accordingly, 
coolants  are  required  to  remove  any  generated  heat,  to  lower  the  temperature  of  the 
cutting  tool,  and,  consequently,  to  increase  the  tool  service  life.  In  order  to  fulfill  such 
conditions  and  function  properly,  a  cutting  fluid  must  possess  certain  characteristics: 

1.  The  cutting  fluid  must  possess  suitable  chemical  properties  (i.e.,  to  be  appropriate 
from  the  point  of  view  of  chemistry),  must  not  react  with  the  workpiece  material 
or  cause  corrosion  in  any  component  of  the  machine  tool,  and  should  not  promote 


9.7  Cutting  Fluids  355 

the  formation  of  rust  or  spoil  the  lubricating  oil  of  the  machine  bearing  and  slides 
whenever  it  comes  in  contact  with  that  oil. 

2.  The  cutting  fluid  must  be  chemically  stable  (i.e.,  must  not  change  its  properties 
with  time). 

3.  No  poisonous  gases  or  fumes  should  evolve  during  machining  so  that  there  is  no 
possibility  of  problems  regarding  the  safety  or  health  of  the  workers. 

4.  The  lubricating  and  cooling  properties  of  the  cutting  fluid  must  be  superior. 

5.  The  fluid  used  should  be  cheap  and  should  be  recycled  by  a  simple  filtration 
process. 

Types  of  Cutting  Fluids 

The  following  discussion  involves  the  different  kinds  of  cutting  fluids  that  are  used  in 
industry  to  satisfy  the  preceding  requirements. 

Pure  oils.  Mineral  oils  such  as  kerosene  or  polar  organic  oils  such  as  sperm  oil,  lin- 
seed oil,  or  turpentine  can  be  used  as  cutting  fluids.  The  application  of  pure  mineral 
oils  is  permissible  only  when  machining  metals  with  high  machinability,  such  as  free 
cutting  steel,  brass,  and  aluminum.  This  is  a  consequence  of  their  poor  lubricating  and 
cooling  properties.  Although  the  polar  organic  oils  possess  good  lubricating  and  cool- 
ing properties,  they  are  prone  to  oxidation,  give  off  unpleasant  odors,  and  tend  to  gum. 

Mixed  oils.  Mineral  oils  are  mixed  with  polar  organic  oils  to  obtain  the  advantages  of 
both  constituents.  In  some  cases,  sulfur  or  chlorine  is  added  to  enable  the  lubricant  to 
adhere  to  the  tool  face,  giving  a  film  of  lubricant  that  is  tougher  and  more  stable.  The 
oils  are  then  referred  to  as  sulfurized  or  chlorinated  oils.  The  chlorinated  oils  have  the 
disadvantage  of  the  possible  emission  of  chlorine  gas  during  the  machining  operation. 

Soluble  oils.  Soluble  oils  are  sometimes  called  water-miscible  fluids  or  emulsifiable 
oils.  By  blending  oil  with  water  and  some  emulsifying  agents,  soapy  or  milky  mix- 
tures can  be  obtained.  These  liquids  have  superior  cooling  properties  and  are  recom- 
mended for  machining  operations  requiring  high  speeds  and  low  pressures. 
Sometimes,  extreme-pressure  additives  are  blended  with  the  mixtures  to  produce 
emulsions  with  superior  lubricating  properties. 

Water  solutions.  A  solution  of  sodium  nitrate  and  trinolamine  in  water  can  be  em- 
ployed as  a  cutting  fluid.  Caustic  soda  is  also  used,  provided  that  the  concentration 
does  not  exceed  5  percent.  If  the  concentration  of  the  solution  exceeds  this  limit,  the 
paint  of  the  machine  and  the  lubricating  oil  of  the  slides  may  be  affected. 

Synthetic  fluids.  Synthetic  fluids  can  be  diluted  with  water  to  give  a  mixture  that 
varies  in  appearance  from  clear  to  translucent.  Extreme-pressure  additives  like  sulfur 
or  chlorine  can  be  added  to  the  mixture  so  that  it  can  be  used  for  difficult  machining 
operations. 


356  9      Physics  of  Metal  Cutting 


HATTER  PHENOMENON 


When  we  feel  cold  in  winter,  our  jaws  and  teeth  may  start  to  chatter.  A  similar  phe- 
nomenon occurs  when  the  cutting  tool  and  workpiece  are  exposed  to  certain  unfavor- 
able cutting  conditions  and  dynamic  characteristics  of  the  machine  tool  structure.  The 
analysis  of  this  chatter  phenomenon  is  an  extremely  complex  task.  However,  thanks  to 
the  work  of  the  late  Professor  Stephen  A.  Tobias  of  the  University  of  Birmingham  in 
England,  we  are  able  to  understand  how  vibrations  of  the  cutting  tool  initiate  and  how 
they  can  be  minimized.  Left  without  remedy,  these  vibrations  result  in  breakage  of  the 
cutting  tool  (especially  if  it  is  ceramic  or  carbide)  and  poor  surface  quality.  They  may 
also  cause  breakage  of  the  entire  machine  tool.  Two  basic  types  of  vibrations  are  gen- 
erated during  machining:  forced  vibrations  and  self-excited  vibrations. 

Forced  vibrations  take  place  as  a  result  of  periodic  force  applied  within  the  ma- 
chine tool  structure.  This  force  can  be  due  to  an  imbalance  in  any  of  the  machine  tool 
components  or  interrupted  cutting  action,  such  as  milling,  in  which  there  is  a  periodic 
engagement  and  disengagement  between  the  cutting  edges  and  the  workpiece.  The  fre- 
quency of  these  forced  vibrations  must  not  be  allowed  to  come  close  to  the  natural  fre- 
quency of  the  machine  tool  system  or  any  of  its  components;  otherwise,  resonance 
(vibrations  with  extremely  high  amplitude)  takes  place.  The  remedy  in  this  case  is  to 
try  to  identify  any  possible  source  for  the  imbalance  of  the  machine  tool  components 
and  eliminate  it.  In  milling  machines,  the  stiffness  and  the  damping  characteristics  of 
the  machine  tool  are  controlled  so  as  to  keep  the  forcing  frequency  away  from  the  nat- 
ural frequency  of  any  component  and/or  the  natural  frequency  of  the  system. 

Self-excited  vibrations,  or  chatter,  occur  when  an  unexpected  disturbing  force, 
such  as  a  hard  spot  in  the  workpiece  material  or  sticking  friction  at  the  chip-tool  inter- 
face, causes  the  cutting  tool  to  vibrate  at  a  frequency  near  the  natural  frequency  of  the 
machine  tool.  As  a  result,  resonance  takes  place,  and  a  minimum  excitation  produces 
an  extremely  large  amplitude.  Such  conditions  drastically  reduce  tool  life,  result  in 
poor  surface  quality,  and  may  cause  damage  to  either  the  workpiece  or  the  machine 
tool  or  both.  This  unfavorable  condition  can  be  eliminated,  or  at  least  reduced,  by  con- 
trolling the  stiffness  and  the  damping  characteristics  of  the  system.  This  is  usually 
achieved  by  selecting  the  proper  material  for  the  machine  bed  (cast  iron  has  better 
damping  characteristics  than  steel),  by  employing  dry-bolted  joints  as  energy  dissipa- 
tors  where  the  vibration  energy  is  absorbed  in  friction,  or  by  using  external  dampers  or 
absorbers.  Advanced  research  carried  out  at  the  University  of  Birmingham  in  England 
indicated  the  potentials  of  employing  layers  of  composites  as  a  means  to  safeguard 
against  the  occurrence  of  chatter. 


[CONOMICS  OF  METAL  CUTTING 

Our  goal  now  is  to  find  out  the  operating  conditions  (mainly  the  cutting  speed)  that 
maximize  the  metal-removal  rate  or  the  tool  life.  These  two  variables  are  in  opposition 
to  each  other;  a  higher  metal-removal  rate  results  in  a  shorter  tool  life.  Therefore,  some 


9.9  Economics  of  Metal  Cutting 


357 


FIGURE    9.19 

Relationship  between 
cost  per  piece  and 
cutting  speed 


Tool-change 
cost 


Optimum  cutting 

speed  for  minimum 

cost/piece 


Cutting  speed 


trade-off  or  balance  must  be  made  in  order  to  achieve  either  minimum  machining  cost 
per  piece  or  maximum  production  rate,  whichever  is  necessitated  by  the  production 

requirements. 

Figure  9.19  indicates  how  to  construct  the  relationship  between  the  cutting 
speed  and  the  total  cost  per  piece  for  a  simple  turning  operation.  The  total  cost  is 
composed  of  four  components:  machining  cost,  idle-time  (nonproductive)  cost,  tool 
cost,  and  tool-change  cost.  An  increase  in  cutting  speed  obviously  results  in  a  re- 
duction in  machining  time  and,  therefore,  lower  machining  cost.  This  is  accompa- 
nied by  a  reduction  in  tool  life,  thus  increasing  tool  and  tool-change  costs.  As  can 
be  seen  in  Figure  9.19,  the  curve  of  the  cost  per  piece  versus  the  cutting  speed  has 
a  minimum  that  corresponds  to  the  optimum  cutting  speed  for  the  minimum  cost 

per  piece. 

The  relationship  between  the  production  time  per  piece  and  the  cutting  speed  can 
be  constructed  in  the  same  manner,  as  shown  in  Figure  9.20.  There  is  also  a  minimum 
for  this  curve  that  corresponds  to  the  optimum  cutting  speed  for  the  maximum  pro- 
ductivity (minimum  time  per  piece).  Usually,  this  value  is  higher  than  the  maximum 
economy  speed  given  in  Figure  9.19.  Obviously,  a  cutting  speed  between  these  two 
limits  (and  depending  upon  the  goals  to  be  achieved)  is  recommended. 


358 


9      Physics  of  Metal  Cutting 


FIGURE    9.20 

Relationship  between 
production  time  per 
piece  and  cutting  speed 


Tool -change 
time  per  piece 


Idle  time 


.Optimum  cutting  speed  for 
j  maximum  production 


Cutting  speed 


Review  Questions 


1.  How  can  the  complex  process  of  metal  cutting 
be  approached? 

2.  Define  the  rake  angle  and  the  clearance  angle 
in  two-dimensional  cutting. 

3.  Why  are  the  angles  in  Question  2  required? 

4.  What  is  the  upper  surface  of  the  tool  called? 

5.  What  is  the  lower  surface  of  the  tool  called? 

6.  What  are  the  cutting  variables  that  affect  the 
values  of  the  rake  and  clearance  angles? 

7.  List  some  drawbacks  if  the  cutting  angles  are 
not  properly  chosen. 

8.  When  should  the  rake  angle  be  taken  as  a  posi- 
tive value? 

9.  When  should  the  rake  angle  be  taken  as  a  nega- 
tive value? 

10.  Can  orthogonal  cutting  actually  take  place? 
Explain. 

11.  Use  sketches  to  explain  the  stages  involved  in 
the  formation  of  chips  during  machining. 


12.  Use  sketches  to  illustrate  the  different  types  of 
machining  chips  and  explain  when  and  why  we 
can  expect  to  have  each  of  these  types. 

13.  Explain  the  stages  involved  in  the  formation  of 
the  built-up  edge. 

14.  Does  the  built-up  edge  have  useful  or  harmful 
effects? 

15.  What  is  meant  by  the  shear  angle? 

16.  What  is  meant  by  the  cutting  ratio? 

17.  Derive  an  expression  for  the  shear  strain  that 
takes  place  during  orthogonal  cutting. 

18.  Draw  a  sketch  of  the  cutting  force  diagram  pro- 
posed by  Ernst  and  Merchant. 

19.  How  can  the  relationship  between  the  shear  and 
rake  angles  be  expressed  according  to  Ernst  and 
Merchant? 

20.  On  what  basis  have  Lee  and  Shaffer  developed 
their  theory? 


Chapter  9  Problems 


359 


21. 
22. 
23. 

24. 
25. 

26. 
27. 


28. 
29. 


Derive  an  expression  for  the  specific  energy 
during  two-dimensional  cutting. 

Illustrate  the  difference  between  orthogonal  and 
oblique  cutting. 

What  are  the  components  of  the  cutting  force  in 
oblique  cutting?  How  do  you  compare  their 
magnitudes  with  each  other? 

Define  the  unit  horsepower. 

Describe  fully  the  geometry  of  single-point  cut- 
ting tools. 

Explain  the  effect  of  each  of  the  cutting  angles 
in  oblique  cutting  on  the  mechanics  of  the 
process. 

List  the  different  cutting  tool  materials  and  enu- 
merate the  advantages,  disadvantages,  and  ap- 
plications of  each. 

What  are  the  two  main  causes  for  tool  wear? 

List  the  different  kinds  of  tool  wear. 


30.  Define  tool  life. 


31.  What  is  the  relationship  between  tool  life  and 
cutting  speed? 

32.  Define  machinability  and  explain  how  it  is 
quantitatively  expressed  by  the  machinability 
index. 

33.  What  are  the  necessary  characteristics  of  cut- 
ting fluids? 

34.  List  the  different  types  of  cutting  fluids  and 
provide  the  advantages  and  limitations  of  each. 

35.  What  are  the  causes  for  forced  vibrations  dur- 
ing machining? 

36.  How  can  forced  vibrations  be  minimized? 

37.  What  is  chatter  and  why  does  it  occur? 

38.  How  can  we  eliminate  chatter? 

39.  What  trouble  can  vibrations  cause  during  ma- 
chining? 

40.  Use  sketches  to  explain  how  the  value  of 
the  optimum  cutting  speed  can  be  obtained 
for  maximum  economy  and  for  maximum 
productivity. 


Problems 


•o, 


MM 




3. 


In  a  turning  operation,  the  diameter  of  the  work- 
piece  is  2  inches  (50  mm),  and  it  rotates  at  360 
revolutions  per  minute.  How  long  will  a  carbide 
tool  last  (n  =  0.3)  under  such  conditions  if  an 
identical  carbide  tool  lasted  for  1  minute  when 
used  at  1000  feet  per  minute  (305.0  m/min.)? 

Determine  the  increase  in  the  tool  life  of  a  car- 
bide tip  as  a  result  of  a  decrease  in  the  cutting 
speed  of  25,  50,  and  75  percent. 
When  turning  a  thin  tube  at  its  edge,  the  follow- 
ing conditions  were  observed: 


Depth  of  cut: 
Chip  thickness: 
Back  rake  angle: 
Cutting  speed: 


0.125  inch 

0.15  inch 

8° 

300  ft/min. 


Calculate  the 

a.  Cutting  ratio 

b.  Shear  angle 

c.  Chip  velocity 

A  geared-head  lathe  is  employed  for  machining 
steel  AISI  1055,  BHN  250.  The  cutting  speed  is 
400  feet  per  minute,  and  the  rate  of  metal  re- 
moval is  2.4  cubic  inches  per  minute.  If  the  tool 
used  has  the  character  0-7-7-7-15-15-1/32,  esti- 
mate the  following: 

The  energy  consumed  in  machining  per  unit 
time 


lime 

The  power  required  at  the  motor 

The  tangential  component  of  the  cutting 

force 


360 


Physics  of  Metal  Cutting 


Neglect  the  correction  factor  for  the  undeformed 
chip  thickness. 

5.  A  5-hp,  2-V,  belt-driven  lathe  is  to  be  used  for 
machining  brass  under  the  following  conditions: 


Cutting  speed: 

Rate  of  metal  removal: 

600  ft/min. 
7.2  in.7min 

SCEA  of  the  tool: 

30° 

Neglect  the  effect  of  chip  thickness.  Does  this 
lathe  have  enough  power  for  the  required  job? 


Design  Prpiecl„ 


± 


Prepare  a  computer  program  that  determines  the  optimum  cutting  speed  that  results  in 
maximum  productivity.  The  program  should  be  interactive,  the  input  being  workpiece 
material,  tool  material,  and  depth  of  cut.  Assume  the  time  for  changing  the  tool  is  60 
seconds  and  the  time  to  return  the  tool  to  the  beginning  of  the  cut  is  20  seconds.  Take 
the  workpiece  material  to  be 

a.  Steel  1020 

b.  Brass 

c.  Aluminum 

d.  Stainless  steel 


Chapter  10 


achlnlng 
Metals 


INTRODUCTION 

This  chapter  will  focus  on  the  technological  aspects  of  the  different  machining 
operations,  as  well  as  the  design  features  of  the  various  machine  tools  em- 
ployed to  perform  those  operations.  In  addition,  the  different  shapes  and 
geometries  produced  by  each  operation,  the  tools  used,  and  the  work-holding 
devices  will  be  covered.  Special  attention  will  be  given  to  the  required  workshop 
calculations  that  are  aimed  at  estimating  machining  parameters  such  as  cut- 
ting speeds  and  feeds,  metal-removal  rate,  and  machining  time. 

Machine  tools  are  designed  to  drive  the  cutting  tool  in  order  to  produce  the 
desired  machined  surface.  For  such  a  goal  to  be  accomplished,  a  machine  tool 
must  include  appropriate  elements  and  mechanisms  capable  of  generating  the 
following  motions: 

1.  A  relative  motion  between  the  cutting  tool  and  the  workpiece  in  the  direc- 
tion of  cutting 

2.  A  motion  that  enables  the  cutting  tool  to  penetrate  into  the  workpiece  until 
the  desired  depth  of  cut  is  achieved 

3.  A  feed  motion  that  repeats  the  cutting  action  every  round  or  every  stroke 
to  ensure  continuation  of  the  cutting  operation 


361 


362  10      Machining  of  Metals 


10.1  TURNING  OPERATIONS 


The  Lathe  and  Its  Construction 

A  lathe  is  a  machine  tool  used  for  producing  surfaces  of  revolution  and  flat  edges. 
Based  on  their  purpose,  construction,  number  of  tools  that  can  simultaneously  be 
mounted,  and  degree  of  automation,  lathes,  or  more  accurately,  lathe-type  machine 
tools,  can  be  classified  as  follows: 

1.  Engine  lathes 

2.  Toolroom  lathes 

3.  Turret  lathes 

4.  Vertical  turning  and  boring  mills 

5.  Automatic  lathes 

6.  Special-purpose  lathes 

In  spite  of  the  diversity  of  lathe-type  machine  tools,  there  are  common  features  with 
respect  to  construction  and  principles  of  operation.  These  features  can  be  illustrated  by 
considering  the  commonly  used  representative  type,  the  engine  lathe,  which  is  shown 
in  Figure  10.1.  Following  is  a  description  of  each  of  the  main  elements  of  an  engine 
lathe. 

Lathe  bed.  The  lathe  bed  is  the  main  frame,  a  horizontal  beam  on  two  vertical  sup- 
ports. It  is  usually  made  of  gray  or  nodular  cast  iron  to  damp  vibrations  and  is  made 
by  casting.  It  has  guideways  that  allow  the  carriage  to  slide  easily  lengthwise.  The 
height  of  the  lathe  bed  should  be  such  that  the  technician  can  do  his  or  her  job  easily 
and  comfortably. 

Headstock.  The  headstock  assembly  is  fixed  at  the  left-hand  side  of  the  lathe  bed  and 
includes  the  spindle,  whose  axis  is  parallel  to  the  guideways  (the  slide  surface  of  the 
bed).  The  spindle  is  driven  through  the  gearbox,  which  is  housed  within  the  headstock. 
The  function  of  the  gearbox  is  to  provide  a  number  of  different  spindle  speeds  (usually 
6  to  18  speeds).  Some  modern  lathes  have  headstocks  with  infinitely  variable  spindle 
speeds  and  that  employ  frictional,  electrical,  or  hydraulic  drives. 

The  spindle  is  always  hollow  (i.e.,  it  has  a  through  hole  extending  lengthwise). 
Bar  stocks  can  be  fed  through  the  hole  if  continuous  production  is  adopted.  Also,  the 
hole  has  a  tapered  surface  to  allow  the  mounting  of  a  plain  lathe  center,  such  as  the  one 
shown  in  Figure  10.2.  It  is  made  of  hardened  tool  steel.  The  part  of  the  lathe  center  that 
fits  into  the  spindle  hole  has  a  Morse  taper,  while  the  other  part  of  the  center  is  coni- 
cal with  a  60°  apex  angle.  As  explained  later,  lathe  centers  are  used  for  mounting  long 
workpieces.  The  outer  surface  of  the  spindle  is  threaded  to  allow  the  mounting  of  a 
chuck,  a  faceplate,  or  the  like. 

Tailstock.  The  tailstock  assembly  consists  basically  of  three  parts:  its  lower  base,  an 
intermediate  part,  and  the  quill.  The  lower  base  is  a  casting  that  can  slide  on  the  lathe 


10.1  Turning  Operations 


363 


FIGURE    10.1 

An  engine  lathe 
(Courtesy  of  Clausing 
Industrial,  Inc., 
Kalamazoo,  Michigan) 


Headstock  assembly 


Tool  post 
Compound  rest 


Center 
Tailstock  quill 


Tailstock  assembly 
Bed 
Lead  screw 
Feed  rod 


bed  along  the  guideways,  and  it  has  a  clamping  device  so  that  the  entire  tailstock  can 
be  locked  at  any  desired  location,  depending  upon  the  length  of  the  workpiece.  The  in- 
termediate part  is  a  casting  that  can  be  moved  transversely  so  that  the  axis  of  the  tail- 
stock  can  be  aligned  with  that  of  the  headstock.  The  third  part,  called  the  quill,  is  a 
hardened  steel  tube  that  can  be  moved  longitudinally  in  and  out  of  the  intermediate 
part  as  required.  This  is  achieved  through  the  use  of  a  handwheel  and  a  screw,  around 
which  a  nut  fixed  to  the  quill  is  engaged.  The  hole  in  the  open  side  of  the  quill  is  ta- 
pered to  allow  the  mounting  of  lathe  centers  or  other  tools  like  twist  drills  or  boring 
bars.  The  quill  can  be  locked  at  any  point  along  its  travel  path  by  means  of  a  clamping 
device. 

Carriage.  The  main  function  of  the  carriage  is  to  mount  the  cutting  tools  and  gener- 
ate longitudinal  and /or  cross  feeds.  It  is  actually  an  H-shaped  block  that  slides  on  the 
lathe  bed  between  the  headstock  and  tailstock  while  being  guided  by  the  V-shaped 


FIGURE    10.2 

A  plain  lathe  center 


Taper 


364  10      Machining  of  Metals 


guideways  of  the  bed.  The  carriage  can  be  moved  either  manually  or  mechanically  by 
means  of  the  apron  and  either  the  feed  rod  or  the  lead  screw. 

The  apron  is  attached  to  the  saddle  of  the  carriage  and  serves  to  convert  the  rotary 
motion  of  the  feed  rod  (or  lead  screw)  into  linear  longitudinal  motion  of  the  carriage 
and,  accordingly,  the  cutting  tool  (i.e.,  it  generates  the  axial  feed).  The  apron  also  pro- 
vides powered  motion  for  the  cross  slide  located  on  the  carriage.  Usually,  the  tool  post 
is  mounted  on  the  compound  rest,  which  is,  in  turn,  mounted  on  the  cross  slide.  The 
compound  rest  is  pivoted  around  a  vertical  axis  so  that  the  tools  can  be  set  at  any  de- 
sired angle  with  respect  to  the  axis  of  the  lathe  (and  that  of  the  workpiece).  These  var- 
ious components  of  the  carriage  form  a  system  that  provides  motion  for  the  cutting 
tool  in  two  perpendicular  directions  during  turning  operations. 

When  cutting  screw  threads,  power  is  provided  from  the  gearbox  to  the  apron  by 
the  lead  screw.  In  all  other  turning  operations,  it  is  the  feed  rod  that  drives  the  carriage. 
The  lead  screw  goes  through  a  pair  of  half  nuts  that  are  fixed  to  the  rear  of  the  apron. 
When  actuating  a  certain  lever,  the  half  nuts  are  clamped  together  and  engage  with  the 
rotating  lead  screw  as  a  single  nut  that  is  fed,  together  with  the  carriage,  along  the  bed. 
When  the  lever  is  disengaged,  the  half  nuts  are  released  and  the  carriage  stops.  On  the 
other  hand,  when  the  feed  rod  is  used,  it  supplies  power  to  the  apron  through  a  worm 
gear.  This  gear  is  keyed  to  the  feed  rod  and  travels  with  the  apron  along  the  feed  rod, 
which  has  a  keyway  extending  along  its  entire  length.  A  modern  lathe  usually  has  a 
quick-change  gearbox  located  under  the  headstock  and  driven  from  the  spindle  through 
a  train  of  gears.  It  is  connected  to  both  the  feed  rod  and  the  lead  screw  so  that  a  vari- 
ety of  feeds  can  easily  and  rapidly  be  selected  by  simply  shifting  the  appropriate 
levers.  The  quick-change  gearbox  is  employed  in  plain  turning,  facing,  and  thread- 
cutting  operations.  Because  the  gearbox  is  linked  to  the  spindle,  the  distance  that  the 
apron  (and  the  cutting  tool)  travels  for  each  revolution  of  the  spindle  can  be  controlled 
and  is  referred  to  as  the  feed. 

The  Turret  Lathe 

A  turret  lathe  is  similar  to  an  engine  lathe,  except  that  the  conventional  tool  post  is  re- 
placed with  a  hexagonal  (or  octagonal)  turret  that  can  be  rotated  around  a  vertical  axis 
as  required.  Appropriate  tools  are  mounted  on  the  six  (or  eight)  sides  of  the  turret.  The 
length  of  each  tool  is  adjusted  so  that,  by  simply  indexing  the  turret,  any  tool  can  be 
brought  into  the  exactly  desired  operating  position.  These  cutting  tools  can,  therefore, 
be  employed  successively  without  the  need  for  dismounting  the  tool  and  mounting  a 
new  one  each  time,  as  is  the  case  with  conventional  engine  lathes.  This  results  in  an 
appreciable  saving  in  the  time  required  for  setting  up  the  tools.  Also,  on  a  turret  lathe, 
a  skilled  machinist  is  required  only  initially  to  set  up  the  tools.  A  laborer  with  limited 
training  can  operate  the  turret  lathe  thereafter  and  produce  parts  identical  to  those  that 
can  be  manufactured  when  a  skilled  machinist  operates  the  lathe.  Figure  10.3  illus- 
trates a  top  view  of  a  hexagonal  turret  with  six  different  tools  mounted  on  its  sides. 
Sometimes,  the  turret  replaces  the  tailstock  and  can  be  either  vertical  (i.e.,  with  a  hor- 
izontal axis)  or  horizontal  (i.e.,  with  a  vertical  axis).  In  this  case,  four  additional  tools 
can  be  mounted  on  the  square  tool  post,  sometimes  called  a  square  turret,  thus  allow- 


10.1  Turning  Operations 


365 


FIGURE    10.3 

Top  view  of  a  hexagonal 
turret  with  six  different 
tools 


Reamer 


ing  twelve  machining  operations  to  be  performed  successively.  Turret  lathes  always 
have  work-holding  devices  with  quick-release  (and  quick-tightening)  mechanisms. 

Specifying  a  Lathe 

It  is  important  for  a  manufacturing  engineer  to  be  able  to  specify  a  lathe  in  order  to 
place  an  order  or  to  compare  and  examine  contract  bids.  The  specifications  of  a  lathe 
should  involve  data  that  reveal  the  dimensions  of  the  largest  workpiece  to  be  machined 
on  that  lathe.  They  also  must  include  the  power  consumption,  as  well  as  information 
that  is  needed  for  shipping  and  handling.  Table  10.1  indicates  an  example  of  how  to 
specify  a  lathe. 


Tool  Holding 

Tools  for  turning  operations  are  mounted  in  a  toolholder  (tool  post).  On  an  engine 
lathe,  it  is  located  on  the  compound  rest.  More  than  one  cutting  tool  (up  to  four)  can 
be  mounted  in  the  toolholder  in  order  to  save  the  time  required  for  changing  and  set- 
ting up  each  tool  should  only  one  tool  be  mounted  at  a  time.  In  all  turning  operations, 
the  following  conditions  for  holding  the  tools  must  be  fulfilled: 

1.  The  tip  of  the  cutting  edge  must  fully  coincide  with  the  level  of  the  lathe  axis. 
This  can  be  achieved  by  using  the  pointed  edge  of  the  lathe  center  as  a  basis  for 
adjustment,  as  shown  in  Figure  10.4.  Failure  to  meet  this  condition  results  in  a 
change  in  the  values  of  the  cutting  angles  from  the  desired  ones. 

2.  The  centerline  of  the  cutting  tool  must  be  horizontal. 

3.  The  tool  must  be  fixed  tightly  along  its  length  and  not  just  on  two  points. 

4.  A  long  tool-overhang  should  be  avoided  in  order  to  eliminate  any  possibility  for 
elastic  strains  and  vibrations. 


366 


10      Machining  of  Metals 


TABLE    10.1 

Example  of 
specifications  of  a  lathe 


/                                   Model 

Example                     \ 

Maximum  swing  over  bed  (largest  diameter 

12  in.  (300  mm) 

of  workpiece) 

Maximum  swing  over  carriage  (largest 

8  in.  (200  mm) 

diameter  over  carriage) 

Hole  through  spindle 

0.75  in.  (19  mm) 

Height  of  centers 

6  in.  (150  mm) 

Turning  length 

24  in.  (600  mm) 

Thread  on  spindle  nose 

Taper  in  spindle  and  tailstock  sleeves 

3  Morse 

21  spindle  speeds 

20-2000  rev/min. 

Metric  threads 

2-6  mm 

Whitworth 

4-28  teeth 

Feeds  per  revolution 

0.0002-0.008  in.  (0.05-0.2  mm) 

Power  required 

1.6  kW 

Net  weight 

1  ton 

Floor  space  requirement 

64/36/56  in. 

(length/width/height) 

(1600/900/1400  mm) 

Lathe  Cutting  Tools 

The  shape  and  geometry  of  lathe  cutting  tools  depend  upon  the  purpose  for  which  they 
are  employed.  Turning  tools  can  be  classified  into  two  main  groups:  external  cutting 
tools  and  internal  cutting  tools. 

Types  of  tools.    Each  of  these  groups  includes  the  following  types  of  tools: 

1.  Turning  tools.  Turning  tools  can  be  either  finishing  or  rough  turning  tools.  Rough 
turning  tools  have  small  nose  radii  and  are  employed  when  deep  cuts  are  made. 
Finishing  tools  have  larger  nose  radii  and  are  used  when  shallower  cuts  are  made 
in  order  to  obtain  the  final  required  dimensions  with  good  surface  finish.  Rough 
turning  tools  can  be  right-hand  or  left-hand  tools,  depending  upon  the  direction  of 
feed.  They  can  have  straight,  bent,  or  offset  shanks.  Figure  10.5  illustrates  the  dif- 
ferent kinds  of  turning  tools. 


FIGURE    10.4 

A  simple  method  for 
tool  setup 


Workpiece 


10.1  Turning  Operations 


367 


FIGURE    10.5 

Different  kinds  of 
turning  tools 


Right-hand  Left-hand 

Rough  turning  tools 


Broad-nose 


Finishing  tools 


2.  Facing  tools.  Facing  tools  are  employed  in  facing  operations  for  machining  fiat 
side  or  end  surfaces.  As  can  be  seen  in  Figure  10.6,  there  are  tools  for  machining 
both  left  and  right  side  surfaces.  These  side  surfaces  are  generated  through  the  use 
of  cross  feed,  contrary  to  turning  operations,  where  longitudinal  feed  is  used. 

3.  Cutoff  tools.  Cutoff  tools,  which  are  sometimes  called  parting  tools,  serve  to  sepa- 
rate the  workpiece  into  parts  and/or  machine  external  annular  grooves,  as  shown  in 
Figure  10.7. 

4.  Thread-cutting  tools.  Thread-cutting  tools  have  either  triangular,  square,  or  trape- 
zoidal cutting  edges,  depending  upon  the  cross  section  of  the  desired  thread.  Also, 
the  plane  angles  of  these  tools  must  always  be  identical  to  those  of  the  thread 
forms.  Thread-cutting  tools  have  straight  shanks  for  external  thread  cutting  and 
bent  shanks  for  internal  thread  cutting.  Figure  10.8  illustrates  the  different  shapes 
of  thread-cutting  tools. 


FIGURE    10.6 

Different  kinds  of 
facing  tools 


FIGURE    10.7 

Cutoff  tools 


u 


J- 


368 


10      Machining  of  Metals 


FIGURE    10.8 

Different  shapes  of 
thread-cutting  tools 


Triangular 


Square 


Trapezoidal 


5.  Form  tools.  As  shown  in  Figure  10.9,  form  tools  have  edges  specially  manufactured 
to  take  a  form  that  is  opposite  to  the  desired  shape  of  the  machined  workpiece. 

Internal  and  external  tools.  The  types  of  internal  cutting  tools  are  similar  to  those  of 
the  external  cutting  tools.  They  include  tools  for  rough  turning,  finish  turning,  thread 
cutting,  and  recess  machining.  Figure  10.10  illustrates  the  different  types  of  internal 
cutting  tools. 

Carbide  tips.  As  previously  mentioned,  a  high-speed  steel  tool  is  usually  made  in 
the  form  of  a  single  piece,  contrary  to  cemented  carbides  or  ceramics,  which  are 
made  in  the  form  of  tips.  The  tips  are  brazed  or  mechanically  fastened  to  steel 
shanks.  Figure  10.11  shows  an  arrangement  that  includes  a  carbide  tip,  a  chip 
breaker,  a  seat,  a  clamping  screw  (with  a  washer  and  a  nut),  and  a  shank.  As  its  name 
suggests,  the  function  of  a  chip  breaker  is  to  break  long  chips  every  now  and  then, 
thus  preventing  the  formation  of  very  long,  twisted  ribbons  that  may  cause  problems 
during  the  machining  operation.  As  shown  in  Figure  10.12,  the  carbide  tips  (or  ce- 
ramic tips)  have  different  shapes,  depending  upon  the  machining  operations  for 
which  they  are  to  be  employed.  The  tips  can  either  be  solid  or  have  a  central  through 
hole,  depending  upon  whether  brazing  or  mechanical  clamping  is  employed  for 
mounting  the  tip  on  the  shank. 


FIGURE    10.9 

Form  tools 


O" 
^ 


/"~\ 


K-S 


FIGURE    10.10 

Different  types  of 
internal  cutting  tools 


YMM 


Recess  or 
groove  making 


r 


nm\    u 


WM 


Internal 
threading 


10.1  Turning  Operations 


369 


FIGURE    10.11 

A  carbide  tip  fastened 
to  a  toolholder 


Clamp 


FIGURE    10.12 

Different  shapes  of 
carbide  tips 


Round 


Diamond 


Diamond 


Square 


Triangle 


Methods  of  Supporting  Workpieces 
in  Lathe  Operations 

Some  precautions  must  be  taken  when  mounting  workpieces  on  a  lathe  to  ensure 
trouble-free  machining.  They  can  be  summarized  as  follows: 

1.  It  is  recommended  that  an  appropriate  gripping  force  that  is  neither  too  high  nor  too 
low  be  used.  A  high  gripping  force  may  result  in  distortion  of  the  workpiece  after 
the  turning  operation,  whereas  a  low  gripping  force  causes  either  vibration  of  the 
workpiece  or  slip  between  the  workpiece  and  the  spindle  (i.e.,  the  rotational  speed, 
or  rpm,  of  the  workpiece  will  be  lower  than  that  of  the  spindle). 

2.  The  workpiece  must  be  fully  balanced,  both  statically  and  dynamically,  by  em- 
ploying counterweights  and  the  like  if  necessary. 

3.  The  cutting  force  should  not  affect  the  shape  of  the  workpiece  or  cause  any  perma- 
nent deformation.  A  manufacturing  engineer  should  calculate  the  cutting  force  using 
his  or  her  knowledge  of  metal  cutting  (Chapter  9)  and  then  check  whether  or  not  such 
a  force  will  cause  permanent  deformation  by  using  stress  analysis.  Such  calculations 
are  very  important  when  machining  slender  workpieces  (i.e.,  those  with  high  length- 
to-diameter  ratios).  Whenever  it  becomes  evident  that  the  cutting  force  will  cause 
permanent  deformation,  the  machining  parameters  must  be  changed  to  reduce  the 
magnitude  of  the  force  (e.g.,  use  a  smaller  depth  of  cut  or  lower  feed). 

Following  is  a  brief  discussion  of  each  of  the  work-holding  methods  employed  in 
lathe  operations. 

Holding  the  workpiece  between  two  centers.  The  workpiece  is  held  between  two 
centers  when  turning  long  workpieces  like  shafts  and  axles  having  length-to-diameter 
ratios  higher  than  3  or  4.  Before  a  workpiece  is  held,  each  of  its  flat  ends  must  be  pre- 
pared by  drilling  a  60°  center  hole.  The  pointed  edges  of  the  live  center  (mounted  in 
the  tailstock  so  that  its  conical  part  rotates  freely  with  the  workpiece)  and  the  dead 
center  (mounted  in  the  spindle  hole)  are  inserted  in  the  previously  drilled  center  holes. 


370 


10      Machining  of  Metals 


FIGURE    10.13 

Holding  the  workpiece 
between  two  centers 
during  turning 


Faceplate 

(screwed  on  the 

spindle  nose) 


As  shown  in  Figure  10.13,  a  driving  dog  is  clamped  on  the  left  end  of  the  workpiece 
by  means  of  a  tightening  screw.  The  tail  of  the  lathe  dog  enters  a  slot  in  the  driving- 
dog  plate  (or  faceplate),  which  is  screwed  on  the  spindle  nose. 

When  very  long  workpieces  having  length-to-diameter  ratios  of  10  or  more  are 
turned  between  centers,  rests  must  be  used  to  provide  support  and  prevent  sagging  of 
the  workpiece  at  its  middle.  Steady  rests  are  clamped  on  the  lathe  bed  and  thus  do  not 
move  during  the  machining  operation;  follower  rests  are  bolted  to  and  travel  with  the 
carriage.  A  steady  rest  employs  three  adjustable  fingers  to  support  the  workpiece. 
However,  in  high-speed  turning,  the  steady  rest  should  involve  balls  and  rollers  at  the 
end  of  the  fingers  where  the  workpiece  is  supported.  A  follower  rest  has  only  two  fin- 
gers and  supports  the  workpiece  against  the  cutting  tool.  A  steady  rest  can  be  used  as 
an  alternative  to  the  tailstock  for  supporting  the  right-hand  end  of  the  workpiece.  Fig- 
ure 10.14  illustrates  a  steady  rest  used  to  support  a  very  long  workpiece. 

Holding  the  workpiece  in  a  chuck.  When  turning  short  workpieces  and/or  when  per- 
forming facing  operations,  the  workpiece  is  held  in  a  chuck,  which  is  screwed  on  the 
spindle  nose.  A  universal,  self-centering  chuck  has  three  jaws  that  can  be  moved  sep- 


FIGURE    10.14 

A  steady  rest  used  to 
support  a  very  long 
workpiece 


Three  adjustable 
aws 


10.1  Turning  Operations 


371 


arately  or  simultaneously  in  radial  slots  toward  its  center  to  grip  the  workpiece  or  away 
from  its  center  to  release  the  workpiece.  This  movement  is  achieved  by  inserting  a 
chuck  wrench  into  a  square  socket  and  then  turning  it  as  required.  Four-jaw  chucks  are 
also  employed;  these  are  popular  when  turning  complex  workpieces  and  those  having 
asymmetric  shapes.  Magnetic  chucks  (without  jaws)  are  used  to  hold  thin,  fiat  work- 
pieces  for  facing  operations.  There  are  also  pneumatic  and  hydraulic  chucks,  and  they 
are  utilized  for  speeding  up  the  processes  of  loading  and  unloading  the  workpieces. 
Figure  10.15  shows  how  a  workpiece  is  held  in  a  chuck. 

Mounting  the  workpiece  on  a  faceplate.  A  faceplate  is  a  large  circular  disk  with  ra- 
dial plain  slots  and  T-slots  in  its  face.  The  workpiece  can  be  mounted  on  it  with  the 
help  of  bolts,  T-nuts,  and  other  means  of  clamping.  The  faceplate  is  usually  employed 
when  the  workpiece  to  be  gripped  is  large  or  noncircular  or  has  an  irregular  shape  and 
cannot,  therefore,  be  held  in  a  chuck.  Before  any  machining  operation,  the  faceplate 
and  the  workpiece  must  be  balanced  by  a  counterweight  mounted  opposite  to  the 
workpiece  on  the  faceplate,  as  shown  in  Figure  10.16. 

Using  a  mandrel.  Disklike  workpieces  or  those  that  have  to  be  machined  on  both 
ends  are  mounted  on  mandrels,  which  are  held  between  the  lathe  centers.  In  this  case, 
the  mandrel  acts  like  a  fixture  and  can  take  different  forms.  As  Figure  10.17  shows,  a 


FIGURE    10.15 

Holding  the  workpiece 
in  a  chuck 


Chuck 
body 


Chuck 
jaw 


Large  bevel  gear 
with  spiral  scrol 
on  the  other  side 


FIGURE    10.16 

Mounting  the  workpiece 
on  a  faceplate 


Faceplate 


Counterweight 


Workpiece 


372 


10      Machining  of  Metals 


FIGURE    10.17 

Mounting  the  workpiece 
on  a  mandrel 


Flattened 

surface 

(for  the  dog) 


Workpiece 


y;  //  ;;  ;/  //  //  /> 


Lathe 
center 


^ 


Mandrel  having 
invisible  slope 


Tightening 
nut 


mandrel  can  be  a  truncated  conical  rod  with  an  intangible  slope  on  which  the  work- 
piece  is  held  by  the  wedge  action.  A  split  sleeve  that  is  forced  against  a  conical  rod  is 
also  employed.  There  are  also  some  other  designs  for  mandrels. 

Holding  the  workpiece  in  a  chuck  collet.  A  chuck  collet  consists  of  a  three-segment 
split  sleeve  with  an  external  tapered  surface.  The  collet  can  grip  a  smooth  bar  placed 
between  these  segments  when  a  collet  sleeve,  which  is  internally  tapered,  is  pushed 
against  the  external  tapered  surface  of  the  split  sleeve,  as  shown  in  Figure  10.18. 

Lathe  Operations 

The  following  sections  focus  on  the  various  machining  operations  that  can  be  per- 
formed on  a  conventional  engine  lathe.  It  must  be  born  in  mind,  however,  that  modern 
computerized  numerically  controlled  (CNC)  lathes  have  more  capabilities  and  can  do 
other  operations,  such  as  contouring,  for  example.  Following  are  the  conventional 
lathe  operations. 

Cylindrical  turning.  Cylindrical  turning  is  the  simplest  and  the  most  common  of  all 
lathe  operations.  A  single  full  turn  of  the  workpiece  generates  a  circle  whose  center 
falls  on  the  lathe  axis;  this  motion  is  then  reproduced  numerous  times  as  a  result  of  the 
axial  feed  motion  of  the  tool.  The  resulting  machining  marks  are,  therefore,  a  helix 
having  a  very  small  pitch,  which  is  equal  to  the  feed.  Consequently,  the  machined  sur- 
face is  always  cylindrical. 

The  axial  feed  is  provided  by  the  carriage  or  compound  rest,  either  manually  or 
automatically,  whereas  the  depth  of  cut  is  controlled  by  the  cross  slide.  In  roughing 
cuts,  it  is  recommended  that  large  depths  of  cuts,  up  to  1/4  inch  (6  mm)  depending 
upon  the  workpiece  material,  and  smaller  feeds  be  used.  On  the  other  hand,  very  fine 
feeds,  smaller  depths  of  cut,  less  than  0.05  inch  (0.4  mm),  and  high  cutting  speeds  are 


FIGURE    10.18 

Holding  the  workpiece 
in  a  chuck  collet 


Workpiece 


10.1  Turning  Operations 


373 


FIGURE    10.19 

Equations  applicable  to  lathe  operations 


Operation 


Cutting  Speed 


Machining  Time 


Material -removal  Rate 


N  (rpm) 


Turning 
(external) 


V  =  it(D  +2d)N 


fN 
where  L  =  *.workpieCe  +  allowance 
i.e.,  length  of  the  workpiece  plus 

allowance 


MRR   =  tt(0  +  d)N-f-d 


f  (feed) 


Boring 


V  =  nDN 


T-k 


MRR   =  n(D  -  d)N-f-d 


Facing 


Feed,  f 


max.  V  =  nDN 
min.  V  =  0 

..      nDN 

mean  y  =  — - — 


D  +  allowance 
2fN 


max.  MRR   =  nDN-f-d 
mean  MRR   =  


Parting 


max.  V  =  nDN 
min.  V  =  0 

■nDN 
2 


D  +  allowance 
2fN 


mean  1/ 


max.  MRR   =  nDN-f-d 

MDD        irDN-f-d 
mean  MRR   = 


Feed,  f 


preferred  for  finishing  cuts.  Figure  10.19  indicates  the  equations  used  to  estimate  the 
different  machining  parameters  in  cylindrical  turning. 

Facing.  The  result  of  a  facing  operation  is  a  flat  surface  that  is  either  the  entire  end 
surface  of  the  workpiece  or  an  annular  intermediate  surface  like  a  shoulder.  During  a 
facing  operation,  feed  is  provided  by  the  cross  slide,  whereas  the  depth  of  cut  is  con- 
trolled by  the  carriage  or  compound  rest.  Facing  can  be  carried  out  either  from  the  pe- 
riphery inward  or  from  the  center  of  the  workpiece  outward.  It  is  obvious  that  the 
machining  marks  in  both  cases  take  the  form  of  a  spiral.  Usually,  it  is  preferred  to 
clamp  the  carriage  during  a  facing  operation  as  the  cutting  force  tends  to  push  the  tool 
(and,  of  course,  the  whole  carriage)  away  from  the  workpiece.  In  most  facing  opera- 
tions, the  workpiece  is  held  in  a  chuck  or  on  a  faceplate.  Figure  10.19  also  indicates 
the  equations  applicable  to  facing  operations. 


374 


10      Machining  of  Metals 


Groove  cutting.  In  cutoff  and  groove-cutting  operations,  only  cross  feed  of  the  tool  is 
employed.  The  cutoff  and  grooving  tools  that  were  previously  discussed  are  employed. 

Boring  and  internal  turning.  Boring  and  internal  turning  are  performed  on  the  inter- 
nal surfaces  by  a  boring  bar  or  suitable  internal  cutting  tool.  If  the  initial  workpiece  is 
solid,  a  drilling  operation  must  be  performed  first.  The  drilling  tool  is  held  in  the  tail- 
stock,  which  is  then  fed  against  the  workpiece. 

Taper  turning.  Taper  turning  is  achieved  by  driving  the  tool  in  a  direction  that  is  not 
parallel  to  the  lathe  axis  but  inclined  to  it  with  an  angle  that  is  equal  to  the  desired 
angle  of  the  taper.  Following  are  the  different  methods  used  in  taper  turning: 

1.  One  method  is  to  rotate  the  disk  of  the  compound  rest  with  an  angle  equal  to  half  the 
apex  angle  of  the  cone,  as  is  shown  in  Figure  10.20.  Feed  is  manually  provided  by 
cranking  the  handle  of  the  compound  rest.  This  method  is  recommended  for  the  taper 
turning  of  external  and  internal  surfaces  when  the  taper  angle  is  relatively  large. 

2.  Special  form  tools  can  be  used  for  external,  very  short,  conical  surfaces,  as  shown 
in  Figure  10.21.  The  width  of  the  workpiece  must  be  slightly  smaller  than  that  of 
the  tool,  and  the  workpiece  is  usually  held  in  a  chuck  or  clamped  on  a  faceplate.  In 


FIGURE    10.20 

Taper  turning  by 
rotating  the  disk  of  the 
compound  rest 


Workpiece 


Chuck 


FIGURE    10.21 

Taper  turning  by 
employing  a  form  tool 


Workpiece 


Tool 


Chuck 


10.1  Turning  Operations 


375 


this  case,  only  the  cross  feed  is  used  during  the  machining  process,  and  the  carriage 
is  clamped  to  the  machine  bed. 

The  method  of  offsetting  the  tailstock  center,  as  shown  in  Figure  10.22,  is  em- 
ployed for  the  external  taper  turning  of  long  workpieces  that  are  required  to  have 
small  taper  angles  (less  than  8°).  The  workpiece  is  mounted  between  the  two  cen- 
ters; then  the  tailstock  center  is  shifted  a  distance  S  in  the  direction  normal  to  the 
lathe  axis.  This  distance  can  be  obtained  from  the  following  equation: 


5  = 


L(D  -  d) 


(10.11 


where:  L  is  the  full  length  of  the  workpiece 

D  is  the  largest  diameter  of  the  workpiece 
d  is  the  smallest  diameter  of  the  workpiece 
i  is  the  length  of  the  tapered  surface 

4.  A  special  taper-turning  attachment,  such  as  the  one  shown  in  Figure  10.23,  is  used 
for  turning  very  long  workpieces,  when  the  length  is  larger  than  the  full  stroke  of 
the  compound  rest.  The  procedure  followed  in  such  cases  involves  complete  disen- 
gagement of  the  cross  slide  from  the  carriage,  which  is  then  guided  by  the  taper- 
turning  attachment.  During  this  process,  the  automatic  axial  feed  can  be  used  as 
usual.  This  method  is  recommended  for  very  long  workpieces  with  a  small  cone 
angle  (8°  through  10°). 

Thread  cutting.  For  thread  cutting,  the  axial  feed  must  be  kept  at  a  constant  rate, 
which  is  dependent  upon  the  rotational  speed  (rpm)  of  the  workpiece.  The  relationship 
between  both  is  determined  primarily  by  the  desired  pitch  of  the  thread  to  be  cut. 

As  previously  mentioned,  the  axial  feed  is  automatically  generated  when  cutting  a 
thread  by  means  of  the  lead  screw,  which  drives  the  carriage.  When  the  lead  screw  rotates 


FIGURE    10.22 

Taper  turning  by 
offsetting  the  tailstock 
center 


376 


10      Machining  of  Metals 


FIGURE    10.23 

Taper  turning  by 
employing  a  special 
attachment 


^ 


a  single  revolution,  the  carriage  travels  a  distance  equal  to  the  pitch  of  the  lead  screw.  Con- 
sequently, if  the  rotational  speed  of  the  lead  screw  is  equal  to  that  of  the  spindle  (i.e.,  that 
of  the  workpiece),  the  pitch  of  the  resulting  cut  thread  is  exactly  equal  to  that  of  the  lead 
screw.  The  pitch  of  the  resulting  thread  being  cut,  therefore,  always  depends  upon  the  ratio 
of  the  rotational  speeds  of  the  lead  screw  and  the  spindle: 


pitch  of  lead  screw 


rpm  of  workpiece 


desired  pitch  of  workpiece      rpm  of  lead  screw 

=  spindle-to-carriage  gearing  ratio 


(10.2) 


This  equation  is  useful  in  determining  the  kinematic  linkage  between  the  lathe  spindle 
and  the  lead  screw  and  enables  proper  selection  of  the  gear  train  between  them. 

In  thread-cutting  operations,  the  workpiece  can  be  either  held  in  a  chuck  or 
mounted  between  two  lathe  centers  for  relatively  long  workpieces.  The  form  of  the 
tool  used  must  exactly  coincide  with  the  profile  of  the  thread  to  be  cut  (i.e.,  triangular 
tools  must  be  used  for  triangular  threads,  and  so  on). 

Knurling.  Knurling  is  basically  a  forming  operation  in  which  no  chips  are  produced. 
It  involves  pressing  two  hardened  rolls  with  rough  filelike  surfaces  against  the  rotat- 
ing workpiece  to  cause  plastic  deformation  of  the  workpiece  metal,  as  shown  in  Fig- 
ure 10.24.  Knurling  is  carried  out  to  produce  rough,  cylindrical  (or  conical)  surfaces 
that  are  usually  used  as  handles.  Sometimes,  surfaces  are  knurled  just  for  the  sake  of 
decoration,  in  which  case  there  are  different  knurl  patterns  to  choose  from. 

Cutting  Speeds  and  Feeds 

The  cutting  speed,  which  is  usually  given  in  surface  feet  per  minute  (SFM),  is  the 
number  of  feet  traveled  in  the  circumferential  direction  by  a  given  point  on  the  surface 
(being  cut)  of  the  workpiece  in  one  minute.  The  relationship  between  the  surface  speed 


10.1  Turning  Operations 


377 


FIGURE    10.24 

The  knurling  operation 


and  the  rpm  can  be  given  by  the  following  equation: 
SFM  =  kDN        (see  Table  1 0. 1 ) 
where:  D  is  the  diameter  of  the  workpiece  in  feet 
TV  is  the  rpm 

The  surface  cutting  speed  is  dependent  upon  the  material  being  machined  as  well 
as  the  material  of  the  cutting  tool  and  can  be  obtained  from  handbooks  and  informa- 
tion provided  by  cutting-tool  manufacturers.  Generally,  the  SFM  is  taken  as  100  when 
machining  cold-rolled  or  mild  steel,  as  50  when  machining  tougher  metals,  and  as  200 
when  machining  softer  materials.  For  aluminum,  the  SFM  is  usually  taken  as  400  or 
above.  There  are  also  other  variables  that  affect  the  optimal  value  of  the  surface  cut- 
ting speed.  These  include  the  tool  geometry,  the  type  of  lubricant  or  coolant,  the  feed, 
and  the  depth  of  cut.  As  soon  as  the  cutting  speed  is  decided  upon,  the  rotational  speed 
(rpm)  of  the  spindle  can  be  obtained  as  follows: 


/V  = 


SFM 
kD 


(10.3) 


The  selection  of  a  suitable  feed  depends  upon  many  factors,  such  as  the  required 
surface  finish,  the  depth  of  cut,  and  the  geometry  of  the  tool  used.  Finer  feeds  will  pro- 
duce better  surface  finish,  whereas  higher  feeds  reduce  the  machining  time  during  which 
the  tool  is  in  direct  contact  with  the  workpiece.  Therefore,  it  is  generally  recommended 
to  use  high  feeds  for  roughing  operations  and  finer  feeds  for  finishing  operations.  Again, 
recommended  values  for  feeds,  which  can  be  taken  as  guidelines,  are  found  in  hand- 
books and  in  information  booklets  provided  by  cutting-tool  manufacturers. 

Design  Considerations  for  Turning 

When  designing  parts  to  be  produced  by  turning,  the  product  designer  must  consider 
the  possibilities  and  limitations  of  the  turning  operation  as  well  as  the  machining  cost. 
The  cost  increases  with  the  quality  of  the  surface  finish,  with  the  tightness  of  the  tol- 
erances, and  with  the  area  of  the  surface  to  be  machined.  Therefore,  it  is  not  recom- 
mended that  high-quality  surface  finishes  or  tighter  tolerances  be  used  in  the  product 
design  unless  they  are  required  for  the  proper  functioning  of  the  product.  Figure  10.25 


378 


10      Machining  of  Metals 


FIGURE    10.25 

Design  considerations 
for  turning:  (a)  reduce 
area  of  surface  to  be 
machined;  (b)  reduce 
number  of  operations 
required;  (c)  provide 
allowance  for  tool 
clearance;  (d)  opt  for 
machining  external  over 
internal  surfaces; 
(e)  opt  for  through 
boring  over  alternatives 


R^v^^: 


K^ 


ss 


>*■     LJ 


F3 


(a) 


Not  recommended  Preferred 

(b) 


Preferred  Not  recommended 

(d) 


Cover 


Better  design 


Less  recommended 


(e) 


graphically  depicts  some  design  considerations  for  turning.  Here  are  the  guidelines  to 
be  followed: 

1.  Try  to  reduce  the  area  of  the  surfaces  to  be  machined,  especially  when  a  large  num- 
ber of  parts  is  required  or  when  the  surfaces  are  to  mate  with  other  parts  (see  Fig- 
ure 10.25a). 

2.  Try  to  reduce  the  number  of  operations  required  by  appropriate  changes  in  the  de- 
sign (see  Figure  10.25b). 

3.  Provide  an  allowance  for  tool  clearance  between  different  sections  of  a  product  (see 
Figure  10.25c). 

4.  Always  keep  in  mind  that  machining  of  exposed  surfaces  is  easier  and  less  expen- 
sive than  machining  of  internal  surfaces  (see  Figure  10.25d). 


10.2  Shaping  and  Planing  Operations  379 

5.  Remember  that  through  boring  is  easier  and  cheaper  than  other  alternatives  (see 
Figure  10.25e). 


SHAPING  AND  PLANING  OPERATIONS 

Planing,  shaping,  and  slotting  are  processes  for  machining  horizontal,  vertical,  and  in- 
clined flat  surfaces,  slots,  or  grooves  by  means  of  a  lathe-type  cutting  tool.  In  all  these 
processes,  the  cutting  action  takes  place  along  a  straight  line.  In  planing,  the  workpiece 
(and  the  machine  bed)  is  reciprocated,  and  the  tool  is  fed  across  the  workpiece  to  re- 
produce another  straight  line,  thus  generating  a  flat  surface.  In  shaping  and  slotting,  the 
cutting  tool  is  reciprocated,  and  the  workpiece  is  fed  normal  to  the  direction  of  tool 
motion.  The  difference  between  the  latter  two  processes  is  that  the  tool  path  is  hori- 
zontal in  shaping  and  it  is  vertical  in  slotting.  Shapers  and  slotters  can  be  employed  in 
cutting  external  and  internal  keyways,  gear  racks,  dovetails,  and  T-slots.  Shapers  and 
planers  have  become  virtually  obsolete  because  most  shaping  and  planing  operations 
have  been  replaced  by  more  productive  processes  such  as  milling,  broaching,  and  abra- 
sive machining.  The  use  of  shapers  and  planers  is  now  limited  to  the  machining  of 
large  beds  of  machine  tools  and  the  like. 

In  all  three  processes,  there  are  successive  alternating  cutting  and  idle  return 
strokes.  The  cutting  speed  is,  therefore,  the  speed  of  the  tool  (or  the  workpiece)  in  the 
direction  of  cutting  during  the  working  stroke.  The  cutting  speed  may  be  either  con- 
stant throughout  the  working  stroke  or  variable,  depending  upon  the  design  of  the 
shaper  or  planer.  Let  us  now  discuss  the  construction  and  operation  of  the  most  com- 
mon types  of  shapers  and  planers. 

Horizontal  Push-Cut  Shaper 

Construction.  As  can  be  seen  in  Figure  10.26,  a  horizontal  push-cut  shaper  consists 
of  a  frame  that  houses  the  speed  gearbox  and  the  quick-return  mechanism  that  trans- 
mits power  from  the  motor  to  the  ram  and  the  table.  The  ram  travel  is  the  primary  mo- 
tion that  produces  a  straight-line  cut  in  the  working  stroke,  whereas  the  intermittent 
cross  travel  of  the  table  is  responsible  for  the  cross  feed.  The  tool  head  is  mounted  at 
the  front  end  of  the  ram  and  carries  the  clapper  box  toolholder.  The  toolholder  is  piv- 
oted at  its  upper  end  to  allow  the  tool  to  rise  during  the  idle  return  stroke  in  order  not 
to  ruin  the  newly  machined  surface.  The  tool  head  can  be  swiveled  to  permit  the  ma- 
chining of  inclined  surfaces. 

The  workpiece  can  be  either  bolted  directly  to  the  machine  table  or  held  in  a  vise 
or  other  suitable  fixture.  The  cross  feed  of  the  table  is  generated  by  a  ratchet  and  pawl 
mechanism  that  is  driven  through  the  quick-return  mechanism  (i.e.,  the  crank  and  the 
slotted  arm).  The  machine  table  can  be  raised  or  lowered  by  means  of  a  power  screw 
and  a  crank  handle.  It  can  also  be  swiveled  in  a  universal  shaper. 

Quick-return  mechanism.  As  can  be  seen  in  Figure  10.27,  the  quick-return  mecha- 
nism involves  a  rotating  crank  that  is  driven  at  a  uniform  angular  speed  and  an  oscil- 
lating slotted  arm  that  is  connected  to  the  crank  by  a  sliding  block.  The  working  stroke 
takes  up  an  angle  (of  the  crank  revolution)  that  is  larger  than  that  of  the  return  stroke. 


380 


10      Machining  of  Metals 


FIGURE    10.26 

Design  features  of  a 
horizontal  push-cut 
shaper 


Feed  screw 

(to  control 

depth  of  cut) 


Tool  slide 


Clapper 
box 


■Table 


Screw 
for  adjusting 
table  height 


Because  the  angular  speed  of  the  crank  is  constant,  it  is  obvious  that  the  time  taken  by 
the  idle  return  stroke  is  less  than  that  taken  by  the  cutting  stroke.  In  fact,  it  is  the  main 
function  of  the  quick-return  mechanism  to  reduce  the  idle  time  during  the  machining 
operation  to  a  minimum. 

Now,  let  us  consider  the  average  speed  (s)  of  the  tool  during  the  cutting  stroke.  It 
can  be  determined  as  a  function  of  the  length  of  the  stroke  and  the  number  of  strokes 
per  minute  as  follows: 


2LN  .    .  ,    .     ,    .    .    , 

5  = in  tt/min.  (m/min.) 

C 

where:  L  is  the  length  of  stroke  in  feet  (m) 

TV  is  the  number  of  strokes  per  minute 
C  is  the  cutting  time  ratio 

Note  that  the  cutting  time  ratio  is 

cutting  time 


(10.4) 


C  = 


total  time  for  one  crank  revolution 


_  angle  corresponding  to  cutting  stroke 

2k 

It  is  also  obvious  that  the  total  number  of  strokes  required  to  machine  a  given  surface 
can  be  given  by  the  following  equation: 


W_ 
f 


(10.5) 


10.2  Shaping  and  Planing  Operations 


381 


FIGURE    10.27 

Details  and  working 
principles  of  the  quick- 
return  mechanism 


Length  of  stroke 


Sliding 
block 


where:  W  is  the  total  width  of  the  workpiece 

/is  the  cross  feed  (e.g.,  inches  per  stroke) 
Therefore,  the  machining  time  T  is  n/N.  After  mathematical  manipulation,  it  can  be 
given  as  follows: 

2LN  (10.6) 


T  = 


sxC 


382  10      Machining  of  Metals 


Next,  the  metal-removal  rate  (MRR)  can  be  given  by  the  following  equation: 

MRR  =  rx/xLxyV(in.3/min.)  (10.7) 

Vertical  Shaper 

The  vertical  shaper  is  similar  in  construction  and  operation  to  the  push-cut  shaper,  the 
difference  being  that  the  ram  and  the  tool  head  travel  vertically  instead  of  horizontally. 
Also,  in  this  type  of  shaper,  the  workpiece  is  mounted  on  a  round  table  that  can  have 
a  rotary  feed  whenever  desired  to  allow  the  machining  of  curved  surfaces  (e.g.,  spiral 
grooves).  Vertical  shapers,  which  are  sometimes  referred  to  as  slotters,  are  used  in  in- 
ternal cutting.  Another  type  of  vertical  shaper  is  known  as  a  keyseater  because  it  is 
specially  designed  for  cutting  keyways  in  gears,  cams,  pulleys,  and  the  like. 

Planer 

A  planer  is  a  machine  tool  that  does  the  same  work  as  the  horizontal  shaper  but  on  work- 
pieces  that  are  much  larger  than  those  machined  on  a  shaper.  Although  the  designs  of 
planers  vary,  most  common  are  the  double-housing  and  open-side  constructions.  In  a 
double-housing  planer,  two  vertical  housings  are  mounted  at  the  sides  of  the  long,  heavy 
bed.  A  cross  rail  that  is  supported  at  the  top  of  these  housings  carries  the  cutting  tools. 
The  machine  table  (while  in  operation)  reciprocates  along  the  guide  ways  of  the  bed  and 
has  T-slots  in  its  upper  surface  for  clamping  the  workpiece.  In  this  type  of  planer,  the 
table  is  powered  by  a  variable-speed  dc  motor  through  a  gear  drive.  The  cross  rail  can 
be  raised  or  lowered  as  required,  and  the  inclination  of  the  tools  can  be  adjusted  as  well. 
In  an  open-side  planer,  there  is  only  one  upright  housing  at  one  side  of  the  bed.  This  con- 
struction provides  more  flexibility  when  wider  workpieces  are  to  be  machined. 

Planing  and  Shaping  Tools 

Planing  and  shaping  processes  employ  single-point  tools  of  the  lathe  type,  but  heavier 
in  construction.  They  are  made  of  either  high-speed  steel  or  carbon  tool  steel  with  car- 
bide tips.  In  the  latter  case,  the  machine  tool  should  be  equipped  with  an  automatic  lift- 
ing device  to  keep  the  tool  from  rubbing  the  workpiece  during  the  return  stroke,  thus 
eliminating  the  possibility  of  breaking  or  chipping  the  carbide  tips. 

The  cutting  angles  for  these  tools  depend  upon  the  purpose  for  which  the  tool  is 
to  be  used  and  the  material  being  cut.  The  end  relief  angle  does  not  usually  exceed  4°, 
whereas  the  side  relief  varies  between  6°  and  14°.  The  side  rake  angle  also  varies  be- 
tween 5°  (for  cast  iron)  and  15°  (for  medium-carbon  steel). 


DRILLING  OPERATIONS 


Drilling  involves  producing  through  or  blind  holes  in  a  workpiece  by  forcing  a  tool 
that  rotates  around  its  axis  against  the  workpiece.  Consequently,  the  range  of  cutting 
from  this  axis  of  rotation  is  equal  to  the  radius  of  the  required  hole.  In  practice,  two 
symmetrical  cutting  edges  that  rotate  about  the  same  axis  are  employed. 


10.3  Drilling  Operations 


383 


Drilling  operations  can  be  carried  out  by  using  either  hand  drills  or  drilling  ma- 
chines. The  latter  differ  in  size  and  construction.  Nevertheless,  the  tool  always  rotates 
around  its  axis  while  the  workpiece  is  kept  firmly  fixed.  This  is  contrary  to  drilling  on 
a  lathe. 

Cutting  Tools  for  Drilling  Operations 

In  drilling  operations,  a  cylindrical  rotary-end  cutting  tool,  called  a  drill,  is  employed. 
The  drill  can  have  one  or  more  cutting  edges  and  corresponding  flutes  that  are  straight 
or  helical.  The  function  of  the  flutes  is  to  provide  outlet  passages  for  the  chips  gener- 
ated during  the  drilling  operation  and  also  to  allow  lubricants  and  coolants  to  reach  the 
cutting  edges  and  the  surface  being  machined.  Following  is  a  survey  of  the  commonly 
used  types  of  drills. 

Twist  drill.  The  twist  drill  is  the  most  common  type  of  drill.  It  has  two  cutting  edges 
and  two  helical  flutes  that  continue  over  the  length  of  the  drill  body,  as  shown  in  Fig- 
ure 10.28.  The  drill  also  consists  of  a  neck  and  a  shank  that  can  be  either  straight  or  ta- 
pered. A  tapered  shank  is  fitted  by  the  wedge  action  into  the  tapered  socket  of  the 
spindle  and  has  a  tang  that  goes  into  a  slot  in  the  spindle  socket,  thus  acting  as  a  solid 
means  for  transmitting  rotation.  Straight-shank  drills  are  held  in  a  drill  chuck  that  is, 
in  turn,  fitted  into  the  spindle  socket  in  the  same  way  as  tapered-shank  drills. 

As  can  be  seen  in  Figure  10.28,  the  two  cutting  edges  are  referred  to  as  the  lips 
and  are  connected  together  by  a  wedge,  which  is  a  chisel-like  edge.  The  twist  drill  also 
has  two  margins  that  allow  the  drill  to  be  properly  located  and  guided  while  it  is  in  op- 
eration. The  tool  point  angle  (TPA)  is  formed  by  the  two  lips  and  is  chosen  based  on 
the  properties  of  the  material  to  be  cut.  The  usual  TPA  for  commercial  drills  is  118°, 
which  is  appropriate  for  drilling  low-carbon  steels  and  cast  irons.  For  harder  and 
tougher  metals,  such  as  hardened  steel,  brass,  and  bronze,  larger  TPAs  (130°  or  140°) 


FIGURE    10.28 

A  twist  drill 


Body 


Tang 


Tool  point 
angle 


Chisel 

edge 

(wedge 


Margin 


Margin 


Flute 


384 


10      Machining  of  Metals 


give  better  performance.  The  helix  angle  of  the  flutes  of  a  twist  drill  ranges  between 
24°  and  30°.  When  drilling  copper  or  soft  plastics,  higher  values  for  the  helix  angle  are 
recommended  (between  35°  and  45°).  Twist  drills  are  usually  made  of  high-speed 
steel,  although  carbide-tipped  drills  are  also  available.  The  sizes  of  twist  drills  used  in 
industrial  practice  range  from  0.01  inch  to  V/i  inches  (0.25  up  to  80  mm). 

Core  drill.  A  core  drill  consists  of  the  chamfer,  body,  neck,  and  shank,  as  shown  in 
Figure  10.29.  This  type  of  drill  may  have  three  or  four  flutes  and  an  equal  number  of 
margins,  which  ensures  superior  guidance,  thus  resulting  in  high  machining  accuracy. 
The  figure  also  shows  that  a  core  drill  has  a  flat  end.  The  chamfer  can  have  three  or 
four  cutting  edges,  or  lips,  and  the  lip  angle  may  vary  between  90°  and  120°.  Core 
drills  are  employed  for  enlarging  previously  made  holes  and  not  for  originating  holes. 
This  type  of  drill  promotes  greater  productivity,  high  machining  accuracy,  and  superior 
quality  of  the  drilled  surfaces. 

Gun  drill.  A  gun  drill  is  used  for  drilling  deep  holes.  All  gun  drills  are  straight-fluted, 
and  each  has  a  single  cutting  edge.  A  hole  in  the  body  acts  as  a  conduit  to  trans- 
mit coolant  under  considerable  pressure  to  the  tip  of  the  drill.  As  can  be  seen  in  Fig- 
ure 10.30,  there  are  two  kinds  of  gun  drills:  the  center-cut  gun  drill  used  for  drilling 
blind  holes  and  the  trepanning  drill.  The  latter  has  a  cylindrical  groove  at  its  center, 
thus  generating  a  solid  core  that  guides  the  tool  as  it  proceeds  during  the  drilling  op- 
eration. 

Spade  drill.  A  spade  drill  is  used  for  drilling  large  holes  of  3V2  inches  (90  mm)  or 
more.  The  design  of  this  type  of  drill  results  in  a  marked  saving  in  tool  cost  as  well  as 
in  a  tangible  reduction  in  tool  weight  that  facilitates  its  ease  of  handling.  Moreover, 
this  drill  is  easy  to  grind.  Figure  10.31  shows  a  spade  drill. 

Saw-type  cutter.  A  saw-type  cutter,  like  the  one  illustrated  in  Figure  10.32,  is  used 
for  cutting  large  holes  in  thin  metal. 

Drills  made  in  combination  with  other  tools.  An  example  is  a  tool  that  involves  both 
a  drill  and  a  tap.  Step  drills  and  drill  and  countersink  tools  are  also  sometimes  used  in 
industrial  practice. 

Cutting  Speeds  and  Feeds  in  Drilling 

We  can  easily  see  that  the  cutting  speed  varies  along  the  cutting  edge.  It  is  always 
maximum  at  the  periphery  of  the  tool  and  is  equal  to  zero  on  the  tool  axis.  Never- 
theless, we  consider  the  maximum  speed  because  it  is  the  one  that  affects  the  tool 


FIGURE    10.29 

A  core  drill 


Chamfer 


TPA 


n   _■  Neck 

Body 1     i  Shank 


Helix 
angle 


10.3  Drilling  Operations 


385 


FIGURE    10.30 

Gun  drills:  (a) 
trepanning  gun  drill;  (b) 
center-cut  gun  drill 


Cutting     Cutting  fluid 
edge  passage 


Shape  of  the 
resulting  hole 


(a) 


(b) 


FIGURE    10.31 

A  spade  drill 


Fastening 
screw 


Diameter 

of  the 

resulting  hole 


FIGURE    10.32 

A  saw-type  cutter 


386 


10      Machining  of  Metals 


wear  and  the  quality  of  the  machined  surface.  The  maximum  speed  must  not  exceed 
the  permissible  cutting  speed,  which  depends  upon  the  material  of  the  workpiece  as 
well  as  the  material  of  the  cutting  tool.  Data  about  permissible  cutting  speeds  in 
drilling  operations  can  be  found  in  handbooks.  The  rotational  speed  of  the  spindle 
can  be  determined  from  the  following  equation: 


N  = 


CS 

kD 


(10.8) 


where:  N  is  the  rotational  speed  of  the  spindle  (rpm) 
D  is  the  diameter  of  the  drill  in  feet  (m) 
CS  is  the  permissible  cutting  speed  in  ft/min.  (m/min.) 

In  drilling  operations,  feeds  are  expressed  in  inches  or  millimeters  per  revolution. 
Again,  the  appropriate  value  of  feed  to  be  used  depends  upon  the  metal  of  the  work- 
piece  and  drill  material  and  can  be  found  in  handbooks.  Whenever  the  production  rate 
must  be  increased,  it  is  advisable  to  use  higher  feeds  rather  than  increase  the  cutting 
speed. 

Other  Types  of  Drilling  Operations 

In  addition  to  conventional  drilling,  there  are  other  operations  that  are  involved  in  the 
production  of  holes  in  industrial  practice.  Following  is  a  brief  description  of  each  of 
these  operations. 

Boring.  Boring  involves  enlarging  a  hole  that  has  already  been  drilled.  It  is  similar  to 
internal  turning  and  can,  therefore,  be  performed  on  a  lathe,  as  previously  mentioned. 
There  are  also  some  specialized  machine  tools  for  carrying  out  boring  operations. 
These  include  the  vertical  boring  mill,  the  jig  boring  machine,  and  the  horizontal  bor- 
ing machine. 

Counterboring.  As  a  result  of  counterboring,  only  one  end  of  a  drilled  hole  is  en- 
larged, as  is  illustrated  in  Figure  10.33a.  This  enlarged  hole  provides  a  space  in  which 
to  set  a  bolt  head  or  a  nut  so  that  it  will  be  entirely  below  the  surface  of  the  part. 

Spot  facing.  Spot  facing  is  performed  to  finish  off  a  small  surface  area  around  the 
opening  of  a  hole.  As  can  be  seen  in  Figure  10.33b,  this  process  involves  removing  a 
minimal  depth  of  cut  and  is  usually  performed  on  castings  or  forgings. 

Countersinking.  As  shown  in  Figure  10.33c,  countersinking  is  done  to  accommodate 
the  conical  seat  of  a  flathead  screw  so  that  the  screw  does  not  appear  above  the  surface 
of  the  part. 


FIGURE    10.33 

Operations  related  to 
drilling: 

(a)  counterboring; 

(b)  spot  facing; 

(c)  countersinking 


10.3  Drilling  Operations 


387 


FIGURE    10.34 

Details  of  a  reamer 


Fluted  section 


Neck 


Shank 


Relief 
angle 


Rake  angle 


Tool  angle 


Cutting  angles 
of  a  tooth 


Reaming.  Reaming  is  actually  a  "sizing"  process,  by  which  an  already  drilled  hole  is 
slightly  enlarged  to  the  desired  size.  As  a  result  of  a  reaming  operation,  a  hole  has  a  very 
smooth  surface.  The  cutting  tool  used  in  this  operation  is  known  as  a  reamer.  As  shown 
in  Figure  10.34,  a  reamer  has  a  fluted  section,  a  neck,  and  a  shank.  The  fluted  section  in- 
cludes four  zones:  the  chamfer,  the  starting  taper,  the  sizing  zone,  and  the  back  taper. 
The  chamfer  or  bevel  encloses  an  angle  that  depends  upon  the  method  of  reaming  and 
the  material  being  cut.  This  is  a  consequence  of  the  fact  that  this  angle  affects  the  mag- 
nitude of  the  axial  reaming  force.  The  larger  the  chamfer  angle,  the  larger  the  required 
reaming  force.  Table  10.2  indicates  some  recommended  values  of  the  chamfer  angle  for 
different  reaming  conditions.  The  starting  taper  is  the  part  of  the  reamer  that  actually  re- 
moves chips.  Figure  10.34  also  shows  that  each  tooth  of  that  part  of  the  reamer  has  a  cut- 
ting edge  as  well  as  rake,  relief,  and  tool  (or  lip)  angles.  The  sizing  zone  guides  the 
reamer  and  smooths  the  surface  of  the  hole.  Finally,  the  back  taper  serves  to  reduce  fric- 
tion between  the  reamer  and  the  newly  machined  surface. 

Reamers  are  usually  made  of  hardened  tool  steel.  Nevertheless,  reamers  that  are 
used  in  mass  production  are  tipped  with  cemented  carbides  in  order  to  increase  the  tool 
life  and  the  production  rate. 

Tapping.  Tapping  is  the  process  of  cutting  internal  threads.  The  tool  used  is  called  a 
tap.  As  shown  in  Figure  10.35,  it  has  a  boltlike  shape  with  four  longitudinal  flutes. 
Made  of  hardened  tool  steel,  taps  can  be  used  for  either  manual  or  machine  cutting  of 


TABLE    10.2 

Recommended  values 
of  the  chamfer  angle  of 
reamers 


Metal  to  Be  Reamed 


Steel 


Cast  Iron 


Soft  Metals 


Manual  reaming 
Machining  reaming 


l°-3° 
8°-10c 


l°-3° 
20°-30c 


l°-3° 
3°-5° 


388 


10      Machining  of  Metals 


FIGURE    10.35 

A  tap 


Chamfer 


Land 


r 


Flute 


Thread 
length 


threads.  In  the  latter  case,  the  spindle  of  the  machine  tool  must  reverse  its  direction  of 
rotation  at  the  end  of  the  cutting  stroke  so  that  the  tap  can  be  withdrawn  without  de- 
stroying the  newly  cut  thread.  When  tapping  is  carried  out  by  hand,  a  set  of  three  taps 
is  used  for  each  desired  threaded  hole  size.  The  three  taps  differ  slightly  in  size,  and 
two  of  them  are  actually  undersized.  The  first  tap  of  the  set  to  be  used  is  always  a  tap- 
per tap;  it  reduces  the  torque  (and,  consequently,  the  power)  required  for  tapping. 

Design  Considerations  for  Drilling 

Figure  10.36  graphically  depicts  some  design  considerations  for  drilling.  Here  are  the 
guidelines  to  be  followed: 

1.  Make  sure  the  centerline  of  the  hole  to  be  drilled  is  normal  to  the  surface  of  the 
part.  This  is  to  avoid  bending  and  breaking  the  tool  during  the  drilling  operation.  As 
previously  mentioned,  the  twist  drill  has  a  chisel  edge  and  not  a  pointed  edge  at  its 
center.  This,  although  it  facilitates  the  process  of  grinding  the  tool,  causes  the  tool 
to  shift  from  the  desired  location  and  makes  it  liable  to  breakage,  especially  if  it  is 
not  normal  to  the  surface  to  be  drilled.  (See  Figure  10.36a  for  examples  of  poor  and 
proper  design  practice  for  drilled  holes.) 

2.  When  tapping  through  holes,  ensure  that  the  tap  will  be  in  the  clear  when  it  appears 
from  the  other  side  of  the  part  (see  Figure  10.36b). 

3.  Remember  that  it  is  impossible  to  tap  the  entire  length  of  a  blind  or  counterbored 
hole  without  providing  special  tool  allowance  (see  Figure  10.36c). 

Classification  of  Drilling  Machines 

Drilling  operations  can  be  carried  out  by  employing  small  portable  machines  or  by 
using  the  appropriate  machine  tools.  These  machine  tools  differ  in  shape  and  size,  but 
they  have  common  features.  For  instance,  they  all  involve  one  or  more  twist  drills,  each 
rotating  around  its  own  axis  while  the  workpiece  is  kept  firmly  fixed.  This  is  contrary 
to  the  drilling  operation  on  a  lathe,  where  the  workpiece  is  held  in  and  rotates  with  the 
chuck.  Following  is  a  survey  of  the  commonly  used  types  of  drilling  machines. 

Bench-type  drilling  machines.  Bench-type  drilling  machines  are  general-purpose, 
small  machine  tools  that  are  usually  placed  on  benches.  This  type  of  drilling  machine 
includes  an  electric  motor  as  the  source  of  motion,  which  is  transmitted  via  pulleys  and 
belts  to  the  spindle,  where  the  tool  is  mounted.  The  feed  is  manually  generated  by  low- 


10.3  Drilling  Operations 


389 


FIGURE    10.36 

Design  considerations 
for  drilling:  (a)  set 
centerline  of  tool 
normal  to  surface  to  be 
drilled;  (b)  ensure  tap  is 
clear  when  it  appears 
from  other  side; 
(c)  provide  allowance 
when  tapping  a  blind 
hole 


To  be  avoided 


Recommended 


Recommended 


Acceptable 


(a) 


To  be  avoided 


a 


(b) 


II 


=  ^  in.  (6  mm) 


(0 


ering  a  lever  handle  that  is  designed  to  lower  (or  raise)  the  spindle.  The  spindle  rotates 
freely  inside  a  sleeve  (which  is  actuated  by  the  lever  through  a  rack-and-pinion  sys- 
tem) but  does  not  rotate  with  the  spindle. 

The  workpiece  is  mounted  on  the  machine  table,  although  a  special  vise  is  some- 
times used  to  hold  the  workpiece.  The  maximum  height  of  a  workpiece  to  be  machined 
is  limited  by  the  maximum  gap  between  the  spindle  and  the  machine  table. 

Upright  drilling  machines.  Depending  upon  the  size,  upright  drilling  machines  can  be 
used  for  light,  medium,  and  even  relatively  heavy  jobs.  A  light-duty  upright  drilling  ma- 
chine is  shown  in  Figure  10.37.  It  is  basically  similar  to  a  bench-type  machine,  the  main 
difference  being  a  longer  cylindrical  column  fixed  to  the  base.  Along  the  column  is  an 
additional  sliding  table  for  fixing  the  workpiece  that  can  be  locked  in  position  at  any  de- 
sired height.  The  power  required  for  this  type  of  machine  is  greater  than  that  for  a  bench- 
type  drilling  machine  as  this  type  is  employed  in  performing  medium-duty  jobs. 


390 


10      Machining  of  Metals 


FIGURE    10.37 

An  upright  drilling 
machine  (Courtesy  of 
Clausing  Industrial, 
Inc.,  Kalamazoo, 
Michigan) 


Spindle 
Drill  chuck 


Capstan 
wheel 


Table 


Column 


Base 


There  are  also  large  drilling  machines  of  the  upright  type.  In  this  case,  the  ma- 
chine has  a  box  column  and  a  higher  power  to  deal  with  large  jobs.  Moreover,  gear- 
boxes are  employed  to  provide  different  rotational  spindle  speeds  as  well  as  axial  feed 
motion,  which  can  be  preset  at  any  desired  rate. 

Multispindle  drilling  machines.  Multispindle  drilling  machines  are  sturdily  con- 
structed and  require  high  power;  each  is  capable  of  drilling  many  holes  simultaneously. 
The  positions  of  the  different  tools  (spindles)  can  be  adjusted  as  desired.  Also,  the  en- 
tire head  (which  carries  the  spindles  and  the  tools)  can  be  tilted  if  necessary.  This  type 
of  drilling  machine  is  used  mainly  for  mass  production  in  jobs  having  many  holes, 
such  as  cylinder  blocks. 

Gang  drilling  machines.  When  several  separate  heads  (each  with  a  single  spindle)  are 
arranged  on  a  single  common  table,  the  machine  tool  is  then  referred  to  as  a  gang 
drilling  machine.  This  type  of  machine  tool  is  particularly  suitable  where  several  op- 
erations are  to  be  performed  in  succession. 


10.3  Drilling  Operations 


391 


Radial  drills.  Radial  drills  are  particularly  suitable  for  drilling  holes  in  large  and 
heavy  workpieces  that  are  inconvenient  to  mount  on  the  table  of  an  upright  drilling 
machine.  As  shown  in  Figure  10.38,  a  radial  drilling  machine  has  a  main  column  that 
is  fixed  to  the  base.  The  cantileverd  guide  arm,  which  carries  the  drilling  head  spindle 
and  tool,  can  be  raised  or  lowered  along  the  column  and  clamped  at  any  desired  posi- 
tion. The  drilling  head  slides  along  the  arm  and  provides  rotary  motion  and  axial  feed 
motion.  The  cantilevered  guide  arm  can  be  swung,  thus  allowing  the  tool  to  be  moved 
in  all  directions  according  to  a  cylindrical  coordinate  system. 

Turret  drilling  machines.  Machine  tools  that  belong  in  the  turret  drilling  machine  cat- 
egory are  either  semiautomatic  or  fully  automatic.  A  common  design  feature  is  that  the 
main  spindle  is  replaced  by  a  turret  that  carries  several  drilling,  boring,  reaming,  and 
threading  tools.  Consequently,  several  successive  operations  can  be  carried  out  with 
only  a  single  initial  setup  and  without  the  need  for  setting  up  the  workpiece  again  be- 
tween operations. 

Automatic  turret  drilling  machines  that  are  operated  by  NC  or  CNC  systems  (see 
Chapter  14)  are  quite  common.  In  this  case,  the  human  role  is  limited  to  the  initial 
setup  and  monitoring.  This  type  of  machine  tool  has  advantages  over  the  gang-type 
drilling  machine  with  respect  to  the  space  required  (physical  size  of  the  machine  tool) 
and  the  number  of  workpiece  setups. 

Deep-hole  drilling  machines.  Deep-hole  drilling  machines  are  special  machines  em- 
ployed for  drilling  long  holes  like  those  of  rifle  barrels.  Usually,  gun-type  drills  are 
used  and  are  fed  slowly  against  the  workpiece.  In  this  type  of  machine  tool,